Folkalist,
I think it depends on if you are a "ground dweller" or a "hammock hanger".
I think 8x8 for the former would be sufficient but most hammock hangers prefer a little more coverage. I have a McCat clone that I made and it has a 12' ridge line.

Any particular reason you want only 8x8 other than weight? Jacks 'R' Better sells an 8x8 and I believe the Hennesy hammocks come with an 8x8 tarp but several folks have commented that they do get some rain coming under.

Get you some 3mil painters plastic and cut out some different sizes and see what works best. It's pretty cheap testing.

Tango