I have a 20" (older) g5 imac. I personally think the 20" screen is a good size. My bf has one of the newer 24" imac's and the screen is HUGE. It's really really nice, and has awesome graphics - but it's too big.
Then again, it'd ultimately depend on what you're gonna use it for. If you're a gamer, the larger screen would be nice.
My bf and i used to play WoW a lot. During that time, I was totally jealous over the larger screen! 😛 For just everyday use, the 20" is the largest I'd want.
Not sure about the 15" or 17" MBP's. I did used to have a Dell 17" widescreen which was also a good size, but i didn't like it in a laptop because it wasn't as portable.