指定値で等倍に変形

選択したものを入力値に合わせて縦横比固定で拡大縮小します。
↓変形のいろいろ
https://www.indesignjs.de/extendscriptAPI/indesign-latest/#TransformationMatrices.html

↓基準点のいろいろ
https://www.indesignjs.de/extendscriptAPI/indesign-latest/#AnchorPoint.html

//幅で合わす
function main(){
var sel=app.activeDocument.selection, WS=prompt ("幅","30");
for(var i=0; i<sel.length; i++){
    var selGB=sel[i].geometricBounds, 
    selW=selGB[3]-selGB[1], selSc=WS/selW,
    myW=app.transformationMatrices.add({horizontalScaleFactor:selSc, verticalScaleFactor:selSc})
    sel[i].transform(CoordinateSpaces.pasteboardCoordinates, AnchorPoint.BOTTOM_CENTER_ANCHOR, myW);//底中央基点
    }
}app.doScript(main, ScriptLanguage.JAVASCRIPT, [], UndoModes.FAST_ENTIRE_SCRIPT);

//高さで合わす
function main(){
var sel=app.activeDocument.selection, HS=prompt ("高さ","50");
for(var i=0; i<sel.length; i++){
    var selGB=sel[i].geometricBounds, 
    selH=selGB[2]-selGB[0], selSc=HS/selH,
    myH = app.transformationMatrices.add({horizontalScaleFactor:selSc, verticalScaleFactor:selSc})
    sel[i].transform(CoordinateSpaces.pasteboardCoordinates, AnchorPoint.BOTTOM_CENTER_ANCHOR, myH);
    }
}app.doScript(main, ScriptLanguage.JAVASCRIPT, [], UndoModes.FAST_ENTIRE_SCRIPT);