Most are a standard size, though some builders use odd shaped ones (e.g. Stone Computers).
A quick check that might help you:Turn the PC on at the wall, and see if your optical mouse lights up when moved. The mouse uses the "always on " 5v supply that the PC uses to switch on via on the ON switch. If it does light up, the ON switch might be faulty. If it doesn't light up at all then I would go for a new PSU as you have already decided.
I use a PSU tester I got from ebay to quickly diagnose PSU faults as they are a right PITA and not worth the risk of opening..... they bite!
PSU testers only cost about £3
