A 250 watt PS unit will power a PC with a modern chip, single HD, AGP,and 2 optical drives. I'm running a 350 watt and am powering multiple usb devices connected to my PC with no problems.
250- 450 watt PSU's all come in different dimensions. You can find a 450 watt that is physically smaller than a 250.
They are just built to case spec like slimline(microATX), midtower, fulltower,and desktop.
To make things easy get a 450 watt that has the correct dimensions.
Sorry, I had to post better advice. My earlier info was too sparse