Unify Multiple Artboard Sizes

Thanks to @CarlosCanto I can used and it seems to work fine.

//  script.name = resizeArtboards_CS4andUp.jsx;
//  script.description = resizes all artboards;
//  script.requirement = one document with at least one artboard;
//  script.parent = carlos canto // 11/4/12;
//  script.elegant = false;

if (app.documents.length > 0) {


        //alert("more than 0");
                    var idoc = app.activeDocument;


        var title = "Resize All Artboards";


        var width = Number(Window.prompt ("Enter New Artboard Width in points", 612, title));
        var height = Number(Window.prompt ("Enter New Artboard Height in points", 792, title));


    for (i=0; i<idoc.artboards.length; i++) {
        var abBounds = idoc.artboards[i].artboardRect;// left, top, right, bottom


        var ableft = abBounds[0]; // 0
        var abtop = abBounds[1]; // 612
        var abwidth = abBounds[2] - ableft; // 792 // width
        var abheight = abtop- abBounds[3]; // 0 // height

        var abctrx = abwidth/2+ableft;
        var abctry = abtop-abheight/2;

        var ableft = abctrx-width/2;
        var abtop = abctry+height/2;
        var abright = abctrx+width/2;
        var abbottom = abctry-height/2;

        idoc.artboards[i].artboardRect = [ableft, abtop, abright, abbottom];
    }
}
 else  {
        alert ("there are no open documents");
}

 
The main advantage of the first script is about details and options available. 
I hope CarlosCanto's version will be updated