canvasResize.jsx

// Photoshop script to resize canvas and export
// You can add this script to Action for automation

doc = app.activeDocument;

// change the color mode to RGB.  Important for resizing GIFs with indexed colors, to get better results
doc.changeMode(ChangeMode.RGB);

// default background color filling in when resizing canvas
var bgColor = new SolidColor();
bgColor.rgb.hexValue = "000000";
app.backgroundColor = bgColor;

// our web export options
var options = new ExportOptionsSaveForWeb();
options.quality = 70;
options.format = SaveDocumentType.PNG;
options.optimized = true; // default
options.includeProfile = false;
options.transparency = false;
options.PNG8 = true; // default
options.colors = 128;
options.colorReduction = ColorReductionType.SELECTIVE; // default
options.dither = Dither.DIFFUSION; // default
options.ditherAmount = 88; //amount of dither. Only valid for diffusion ( default: 100 )


function exportResizedImage(fWidth, fHeight, outputFolderStr) {
    // these are our values for the END RESULT width and height (in pixels) of our image
    // do the resizing.  if height > width (portrait-mode) resize based on height.  otherwise, resize based on width
    if (doc.height > doc.width) {
        doc.resizeImage(null,UnitValue(fHeight,"px"),null,ResampleMethod.BICUBIC);
    } else {
        doc.resizeImage(UnitValue(fWidth,"px"),null,null,ResampleMethod.BICUBIC);
    }
    // Convert the canvas size as informed above for the END RESULT
    app.activeDocument.resizeCanvas(UnitValue(fWidth,"px"),UnitValue(fHeight,"px"));

    var array = doc.path.toString().split("/");
    var currFolder = array[array.length-1];
    var newName = doc.path + outputFolderStr + currFolder;
    var outputFolder = Folder(newName);
    if (!outputFolder.exists) outputFolder.create();
    newName += "/"+doc.name;
    doc.exportDocument(File(newName), ExportType.SAVEFORWEB, options);
}

// Change outputFolderStr to yours
exportResizedImage(454, 454, "/../");
exportResizedImage(390, 390, "/../");