Batch resizing photos of different dimensions

The following script will resize the canvas to square based on the longest edge, effectively "letterboxing" a landscape image or  the opposite on a portrait image. The canvas extension colour is white. You can remove the // comment double slashes from the code to enable resizing, then change the dimensions from 1020,1020 to whatever pixel size you require.

/* https://forums.adobe.com/message/6424562#6424562 */

#target photoshop

// Save the current ruler units and set to pixels
var savedRuler = app.preferences.rulerUnits;
app.preferences.rulerUnits = Units.PIXELS;
 
var backgroundC = new SolidColor();
    backgroundC.rgb.red = 255;
    backgroundC.rgb.green = 255;
    backgroundC.rgb.blue = 255;
    backgroundColor = backgroundC;
   
var doc = app.activeDocument;
 
doc.resizeCanvas(Math.max(doc.width,doc.height),Math.max(doc.width,doc.height))

// Uncomment the line below to fit to a 1020px square rather than longest edge
//doc.resizeImage(1020,1020) 

// Restore the ruler units
app.preferences.rulerUnits = savedRuler;

Actions can not handle sizing of  document well when they have various document Aspect Ratio and sizes.  To size them to a common aspect ratio and size you need to use Photoshop scripting. A  script can crop the images to the output image Aspect Ratio and resample or set the print resolution of the crop to the output image size  so it will have the correct number of pixels and the correct print resolution.

I wrote two Photoshop Plug-In Script Aspect Ratio Slection "AspectRatioSelection.jsx" and Long Side Print Length  "LongSidePrintLength.jsx" You can use in action to enable you to create action that will do what you want to do. They are in my crafting Action Package. The Resolution is changed for the crop the image is not resampled via the plug-in after the aspect ration selection crop is made.

Crafting Actions Package UPDATED Aug 10, 2014 Added Conditional Action steps to Action Palette Tips.
Contains

• Action Actions Palette Tips.txt
• Action Creation Guidelines.txt
• Action Dealing with Image Size.txt
• Action Enhanced via Scripted Photoshop Functions.txt
• CraftedActions.atn Sample Action set includes an example Watermarking action
• Sample Actions.txt Photoshop CraftedActions set saved as a text file.
• More then a dozen Scripts for use in actions

Example
Download

Have a look at the Photoshop Image Processor – File > Scripts > Image Processor