Need Assistance : Creating a script - Using Data Merge info to adjust varying page sizes

Hi @J_Kraft , You might be able to use an idleTask listener along with script labels.

Here my data is:

This example invokes the DataMerge menu item, labels the w & h text frames and adds an idletask:

//need targetengine for listeners
#targetengine "session"
app.scriptPreferences.measurementUnit = MeasurementUnits.INCHES

var mergeDoc, iTask;
var wLabel = "width";
var hLabel = "height";
var wData = "<<W>>";
var hData = "<<H>>";
//a boolean to get the merge document
var isMerge = false;

setLabels();


//invoke the datamerge menu, the Data Merge Panel needs to be open
var menuItem =  app.menuActions.itemByID(108036)
if(menuItem.enabled){ 
    menuItem.invoke();
    //set isMerge to true and listen for the idleEvent
    isMerge = true;
    //make a new idleTask. The task name and the sleep time 4 sec
    iTask = makeIdleTask("onMergeDocument", 4000)
    iTask.addEventListener(IdleEvent.ON_IDLE, onMergeDoc);
}else{
    alert("Open DataMerge panel and try again");
};  


/**
* Idle task function runs when data merge is invoked 
* @param event 
* 
*/
function onMergeDoc(e){
    if (isMerge) {
        //the merged document
        mergeDoc = app.activeDocument;
        iTask.remove();
        //resize the pages based on the w & h fields
        setPages()
    } 
}


/**
* Resize the merged document‘s pages 
* @return void 
*/

function setPages(){
    var pgs = mergeDoc.pages
    var api, nw, nh;

    for (var i = 0; i < pgs.length; i++){
        api = pgs[i].allPageItems;
        //gwet the width and heights
        for (var j = 0; j < api.length; j++){
            if (api[j].label == wLabel) {
                nw = Number(api[j].contents);
            } 
            if (api[j].label == hLabel) {
                nh = Number(api[j].contents);
            }
        }
        //Resize the pages, adjust as needed
        pgs[i].resize (CoordinateSpaces.INNER_COORDINATES,AnchorPoint.CENTER_ANCHOR,ResizeMethods.REPLACING_CURRENT_DIMENSIONS_WITH,[nw*72,nh*72])
    }  
    cleanup();
    
}



/** 
* sets datamerge field’s script label 
* @return void 
* 
*/
function setLabels(){
    var mapi = app.documents[0].allPageItems;

    for (var i = 0; i < mapi.length; i++){
        if (mapi[i].contents == wData) {
            mapi[i].label = wLabel;
        } 
        if (mapi[i].contents == hData) {
            mapi[i].label = hLabel;
        } 
    };
}


/**
* Makes a new named idleTask 
* @param the task name 
* @param the sleep time 
* @return the named idleTask
*/
function makeIdleTask(n, s){
    var it;
    try {
        app.idleTasks.add({name:n, sleep:s})
    }catch(e) {
        it = app.idleTasks.item (n);
    } 
    return app.idleTasks.item (n);
}


/**
* Remove the w & h text frames 
* @return void 
* 
*/
function cleanup(){
    var api = mergeDoc.allPageItems
    for (var i = 0; i < api.length; i++){
        if (api[i].label == wLabel || api[i].label == hLabel) {
            api[i].remove()
        } 
    };   
}

The resized merged pages