I only tried that for elements if The gui and not The Background and for them it worked. The Problem is that they aren't shown outside of The 640x480 box in The center.
So, either this boc in which elements of The gui are shown is set by th Background size, then it'd Work, or it's really hardcoded and The Background wouldn't show either.
Edit: So, I made some tests.
As a short introduction: For each element you set it's upper left corner and it's size (height and width) with those 4 parameters seen in Dasts screenshot.
For the actual GUI elements (that is everything except for the root), you can enter negative values for the coordinates, but only the part of the element that is inside the 640x480 box in the middle will be shown. Changing their size is no problem either.
For the root (aka the background) that's different. You can change its size and it will show outside of that little box in the middle. But if you enter negative coordinates to move the top left corner to the top left of the screen, the entire gui will just disappear and all that remains is the big metallic background. I did not test moving the root to positive coordinates (since that'd be pointless), so I don't know if its coordinates have to be zero or simply positive. I also did not try if other elements are visible on the enlarged background where they wouldn't be visible before.
But it seems like we can't make a bigger menu gui :/