Depends on the size of the hammock. I have a Blackbird - smallest tarp I would use would be the MacCat Standard. Having a few inches over the ends of the hammock and enough side to side coverage, you can pitch close down to the bugnet and stay dry.

Hennessy stock tarps will keep you dry enough, while you're in the hammock. If that's all you want a simple 8x8 pitched in diamond mode might do fine for a hammock of less than 8' length.
_________________________
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki

http://hikeandbackpack.com