What you could do is reacting to the current GUI scale that you can get in the following way:
ScaledResolution scaledresolution = new ScaledResolution(this.mc, this.mc.displayWidth, this.mc.displayHeight);
int guiScale = scaledresolution.getScaleFactor();
You'd just have to alter the size of your quad. It won't work for buttons and the other elements though, unless you are reverting the scale with glScalef which will screw the whole mouse interaction, you'd have to manually call the methods on these then.