MaxCanvas.jsx for Photoshop

    // K Baker @ 14.01.2014 11:21:01
    // Resize canvas to largest size in Photoshop
    // Tested only in Photoshop CC on Maverics

    // Enable double clicking from the Macintosh Finder or the Windows Explorer
    #target photoshop

    // If double clicked, bring app to front
    app.bringToFront();

    if (documents.length > 0) {

            // Get original height and width

            originalWidth = app.activeDocument.width;
            originalHeight = app.activeDocument.height;

            // Resize canvas to the largest size

            if (originalWidth > originalHeight) {

                    newWidth = originalWidth;
                    newHeight = originalWidth;

            } else if (originalHeight > originalWidth) {

                    newWidth = originalHeight;
                    newHeight = originalHeight;

            } else {

                    // There's really nothing to do here, as the height and width aparently are the same size

            }

            app.activeDocument.resizeCanvas(newWidth, newHeight, AnchorPosition.MIDDLECENTER);

    } else {

            alert("You must have at least one open document to run this script!");

    }

    // EOF