I can't give you the dimensions exactly I'm afraid, but I can tell you why WeLoveMacs may have been reluctant to help you out: The PSU in the G5 is under a removable plate which forms the visible 'floor' of the g5 case when you open it up. The plate is actually wider than the PSU, but the PSU is still huge, and to get it out to measure you have to remove the CPUs and often the logic board.
The G5 PSU should fit in a G4 case, but whether it will fit in there with the other components, I doubt. It is a much bigger unit than the G4 MDD which is a similar shape.
You need to ask someone who has one on the shelf really. I fitted one the other day before you posted, or I'd have helped you out. Ask a big Apple service centre.

The Quad and some of the dual core 2.3s have a 1KW PSU (!)