How big should my power supply be?
The answer is: there isn't an answer.
OK, that's not what you wanted to hear. You probably wanted a number. I don't blame you. We all want a number.
But there are a number of things that complicate the issue. First of all, the wattage rating on a power supply (ie, how "big" it is) is not that valuable. It's the combination of a lot of factors, it's not always honest, and even on a good day it's an approximation.
If you want a number, here's a number: 450. A well-made 450 W power supply will run most any modern computer. Just remember that the most important part of that sentence is well-made, and the second most important part is most.
So that's the short answer. If you want a longer answer, then you probably want the Big Power Supply Guide.