I'm with leepingreenlizards, and have only one tarp. 8'x10' DIY. Two colors, for prettiness that I use to justify it's size. And I wish it was 8'x11', for extra coverage on the ends of my hammock.....
Rectangular, not cat-cut. Extra tie-outs in the middle help with tautness, though it's never been a big problem without them. I have stayed dry with it even under EXTREMELY pounding-hard rainy conditions. We had one storm in the BWCA that convinced me never to skimp on tarp size.