There are different sizes, but it's the quality of the PSU that matters more than the size.
The cheap PSUs are rated on their peak usage (the ability to power something for a brief moment in time, and some can't even manage that). The better PSUs are rated on how they can sustain power (the ability to power something cleanly for any real length of time... like when pushing both the CPU and graphics card simultaneously, like when gaming).
The other difference is the level of protection a PSU offers if it were to break. The cheap ones don't offer any real protection, so if it goes pop (and trust me, they do), then all your components are at risk of frying (like your hard drive, with all your por... files). The better models offer more protection, so the chances of a PSU taking another component with it is much reduced - in fact I've never seen it yet.
A decent 400w PSU is
far better than a cheapy 700w one.