I recently got a 5x8 tarp on the cheap ($12!) and have done some backyard testing with it. I like the size, it gives decent coverage (two nights in thunderstorms without getting soaked) and the weight is not bad at all. With six aluminum pegs, three bits of cord, stuffsack and a plastic ground sheet it's right at 1lb 1oz.

I have a 7x10, a 10x10, and a 7x9 as well. The 7x9 is my favorite size all around, but mine leaks terribly.

What's your favorite size? Any odd sizes like an 8x8?