Automatically resizing resolution of the image to set file size in Export As

Outside of Adobe programming something, options are limited. The following script is the best that I can offer for now.

What it does:

Instead of the more common approach of lowering JPEG quality to hit a target size, this script keeps the JPEG quality fixed (90%) and reduces pixel dimensions instead. You may wish to change the 90% value, there is a balance between the quality level and the pixel width/height to achieve a target file size on drive.

• Takes the currently open document and exports it via "Save for Web" as a JPEG at a fixed quality (90).
• If the resulting file is larger than the target size, it deletes that file, shrinks the pixel dimensions by a step percentage (default 10%), and tries again.
• This continues until the file fits under the target size, or until the scale hits a safety floor (default 25% of original dimensions), at which point it gives up and warns the user.
• Each pass is done on a duplicate of the original document, so the original stays untouched, and every resize step scales from the original dimensions rather than compounding the resampling of the previously resized copy.
• Output files are saved with dimensions in the filename (myImage_1200x800px.jpg).

Two modes:

• Interactive (HEADLESS_MODE = false): shows a dialog letting the user set/confirm the target size (KB) and the resize step (%) before running.
• Headless (HEADLESS_MODE = true): skips the dialog and just uses the hardcoded MaxSz value. Intended for batch action automation.

/*
JPEG Resize to Target Size KB
Resizes the pixel dimensions of a saved copy to achieve a targeted file size on drive.
Stephen Marsh
24th July 2026 - v1.0: Initial release
https://community.adobe.com/feature-requests-713/automatically-resizing-resolution-of-the-image-to-set-file-size-in-export-as-1632557
Based on:
https://community.adobe.com/questions-712/slow-operation-of-the-script-1149419
*/

////////////////////
// Set HEADLESS_MODE to true to run without showing the target-size dialog, which is
// useful for batch/automation. When set to true, the script uses MaxSz (as set below)
// as the target size in bytes and skips the dialog.
//
// When HEADLESS_MODE is set to false, the dialog is shown so the user can confirm/override
// the target size interactively.
////////////////////
// User script execution Preference
var HEADLESS_MODE = false;
////////////////////
// Set the target size, i.e. 500000 = max. bytes (500Kb)
var MaxSz = 500000; // Also used as the headless target size, change as needed!
////////////////////

// Check that a document is open before proceeding
if (app.documents.length === 0) {
    alert("No document is open! Please open a document and try again.");
    throw new Error("__CANCEL__");
}

main();

function main() {
      var docRef = activeDocument;
      var outputFolder = docRef.path;
      var currentNameWithoutExtension = docRef.name.replace(/\.[^\.]+$/, '');

      // Points at the most recently saved file on disk, used both to check the resulting file size
      // and to remove that file if it misses the target, before the next (smaller) pass is saved
      // under its own new name.
      var NewfileRef = null;

      ////////////////////
      var qualityFactor = 90; // Fixed JPEG save quality value (quality is no longer reduced - pixel dimensions are reduced instead)
      var resizePct = 10; // Decreasing size step % value (percentage points knocked off the scale each pass)
      var minScalePct = 25; // Safety floor - stop reducing once the scale would drop to/below this % of the original
      var theResampleMethod = ResampleMethod.BICUBICSHARPER; // ResampleMethod.BICUBIC || ResampleMethod.BICUBICSMOOTHER
      ////////////////////

      // Capture the ORIGINAL pixel dimensions once, up front, before any resizing takes place.
      // Every pass below resizes a fresh duplicate from the original image
      var origWidthPx = docRef.width.as("px");
      var origHeightPx = docRef.height.as("px");
      var origResolution = docRef.resolution;

      var iterations = 0; // Tracks the number of Save for Web export passes
      var scalePct = 100; // Current % of the ORIGINAL pixel dimensions used for this pass
      var tempDoc = null; // The disposable, resized duplicate used for exporting

      // Suppress dialogs
      var origDisplayDialogs = app.displayDialogs;
      app.displayDialogs = DialogModes.NO;

      try {
            if (HEADLESS_MODE) {
                  // Headless: skip the dialog entirely, use MaxSz as-is (already in bytes).
            }
            else {
                  // Show the dialog to let the user set/confirm the target size (in KB)
                  // and the resize step (%) used between passes
                  var dialogResult = showTargetSizeDialog(MaxSz, resizePct);

                  if (dialogResult === null) {
                        // User cancelled
                        return;
                  }

                  MaxSz = dialogResult.maxSizeBytes; // MaxSz is stored in bytes
                  resizePct = dialogResult.stepPct;
            }

            // Perform the first Save for Web pass at the original pixel dimensions, on a duplicate
            var pass = duplicateAndResize(scalePct);
            tempDoc = pass.doc;
            NewfileRef = buildFileRef(pass.width, pass.height);
            s4wJPEG(tempDoc, NewfileRef, qualityFactor);
            tempDoc.close(SaveOptions.DONOTSAVECHANGES);
            iterations++;

            // Keep trying to save under MaxSz by shrinking the pixel dimensions
            while (NewfileRef.length > MaxSz) {
                  scalePct = scalePct - resizePct;

                  // If this exceeds the target size - remove it from disk before saving the next,
                  // smaller pass under its own new name.
                  NewfileRef.remove();

                  pass = duplicateAndResize(scalePct);
                  tempDoc = pass.doc;
                  NewfileRef = buildFileRef(pass.width, pass.height);
                  s4wJPEG(tempDoc, NewfileRef, qualityFactor);
                  tempDoc.close(SaveOptions.DONOTSAVECHANGES);
                  iterations++;

                  // The floor
                  if (scalePct <= minScalePct) {
                        alert("The file can't be saved within the desired size at a reasonable pixel dimension (" + minScalePct + "% of the original).");
                        break;  // break the loop whenever the scale has shrunk to the safety floor
                  }
            }

            // End of script notification
            if (!HEADLESS_MODE) {
                  app.beep();
                  alert("Saved File: " + NewfileRef.name + " \r " +
                        "Save Iterations: " + iterations + " \r " +
                        "Final Scale: " + scalePct + "% of original \r " +
                        "File Size: " + Math.round(NewfileRef.length / 1024) + " KB"); // Matches Windows Explorer
            }
      }
      finally {
            // Restore the user's original dialog-display preference
            app.displayDialogs = origDisplayDialogs;
      }


      ///// Functions /////

      function duplicateAndResize(pct) {
            var dup = docRef.duplicate();
            var w, h;

            if (pct < 100) {
                  w = Math.round(origWidthPx * (pct / 100));
                  h = Math.round(origHeightPx * (pct / 100));
                  dup.resizeImage(UnitValue(pct, "%"), UnitValue(pct, "%"), origResolution, theResampleMethod);
            }
            else {
                  w = Math.round(origWidthPx);
                  h = Math.round(origHeightPx);
            }

            return { doc: dup, width: w, height: h };
      }

      function buildFileRef(w, h) {
            return new File(outputFolder + "/" + currentNameWithoutExtension + "_" + w + "x" + h + "px.jpg");
      }

      function s4wJPEG(sourceDoc, FileNm, qualityFactor) {
            var options = new ExportOptionsSaveForWeb();
            options.includeProfile = true;
            options.quality = qualityFactor;
            options.format = SaveDocumentType.JPEG; // Save Format for the file
            sourceDoc.exportDocument(File(FileNm), ExportType.SAVEFORWEB, options);
      }
}

function showTargetSizeDialog(defaultSzBytes, defaultStepPct) {
      var defaultSzKB = Math.round(defaultSzBytes / 1000); // Not 1024!?

      // ScriptUI
      var win = new Window("dialog", "JPEG Resize to Target Size (KB) - v1.0");
      win.orientation = "column";
      win.alignChildren = ["fill", "top"];
      win.spacing = 10;
      win.margins = 16;
      win.preferredSize.width = 250;

      // Panel containing the target size and resize step entry fields
      var panel = win.add("panel", undefined, "");
      panel.orientation = "column";
      panel.alignChildren = ["fill", "top"];
      panel.margins = [15, 20, 15, 15];
      panel.spacing = 10;

      var sizeRow = panel.add("group");
      sizeRow.orientation = "row";
      sizeRow.alignChildren = ["left", "center"];
      sizeRow.add("statictext", undefined, "Max size (KB):");
      var szInput = sizeRow.add("editnumber", undefined, String(defaultSzKB));
      szInput.characters = 5;
      szInput.alignment = ["fill", "center"];

      var stepRow = panel.add("group");
      stepRow.orientation = "row";
      stepRow.alignChildren = ["left", "center"];
      stepRow.add("statictext", undefined, "Resize step (%):");
      var stepInput = stepRow.add("editnumber", undefined, String(defaultStepPct));
      stepInput.characters = 5;
      stepInput.alignment = ["fill", "center"];

      // Button row, right aligned, outside/below the panel
      var btnGroup = win.add("group");
      btnGroup.orientation = "row";
      btnGroup.alignment = ["right", "top"];
      btnGroup.spacing = 10;

      var cancelBtn = btnGroup.add("button", undefined, "Cancel", { name: "cancel" });
      var okBtn = btnGroup.add("button", undefined, "OK", { name: "ok" });

      var result = null;

      okBtn.onClick = function () {
            var val = parseFloat(szInput.text);
            if (isNaN(val) || val <= 0) {
                  alert("Please enter a valid target size in KB (a positive number).");
                  return;
            }

            var stepVal = parseFloat(stepInput.text);
            if (isNaN(stepVal) || stepVal <= 0 || stepVal >= 100) {
                  alert("Please enter a valid resize step (a number greater than 0 and less than 100).");
                  return;
            }

            result = {
                  maxSizeBytes: Math.round(val * 1024), // convert KB back to bytes
                  stepPct: stepVal
            };
            win.close();
      };

      cancelBtn.onClick = function () {
            result = null;
            win.close();
      };

      win.show();

      return result;
}

1. Copy the code text to the clipboard
2. Open a new blank file in a plain-text editor (not in a word processor)
3. Paste the code in
4. Save as a plain text format file – .txt
5. Rename the saved file extension from .txt to .jsx
6. Install or browse to the .jsx file to run (see below)

https://prepression.blogspot.com/2017/11/downloading-and-installing-adobe-scripts.html