Function that takes width and height as arguments and resizes all Artboards in an Illustrator document with those dimensions.

fn-resize-artboards.js

function doResizeArtboards(width, height) {
    if (app.documents.length === 0) {
        alert("There are no open documents.");
        return;
    }
    var theDoc = app.activeDocument;
    try {
        for (i = 0; i < theDoc.artboards.length; i++) {

            /**
             * Bounds are always: [left, top, right, bottom]
             */
            var abBounds = theDoc.artboards[i].artboardRect; 

            /** Legend:
             *  - abctrx   = artboard center x
             *  - abctry   = artboard center y
             *  - abwidth  = artboard width
             *  - abheight = artboard height
             */

            var left     = abBounds[0];
            var top      = abBounds[1];
            var abwidth  = abBounds[2] - left;
            var abheight = top - abBounds[3];

            var abctrx   = abwidth / 2 + left;
            var abctry   = top - abheight / 2;

            var left     = abctrx - width / 2;
            var top      = abctry + height / 2;
            var right    = abctrx + width / 2;
            var bottom   = abctry - height / 2;

            theDoc.artboards[i].artboardRect = [
                left, 
                top, 
                right, 
                abbottom
            ];
        }
    }
    catch (e) { alert(e.message) }
}