If you want smallest possible; hang your hammock, measure the hung length of your hammock, add about 6" to that length and with that number you can calculate the diagonal of a rectangle to fit.
For example; I just recently did a review on my new BIAS wwm 11ft and while hanging it found out that a 6x8 tarp will cover it. That includes the roughly 2" inches that is taken off each side for seams ($6 tarp from my bx). I will say that if there were any wind I'd get wet, but it would cover if it were straight down. I have just a few inches on each side and would need drippers within a couple of inches of my hammock ends. I will probably never use this tarp in this climate with this hammock but if I'm feelin lucky I might. Good luck, and as always hang safe.
"We do not go to the green woods and crystal waters to rough it, we go to smooth it."- G. W. Sears
My forum name is Fish<><; I'm in the navy; and I hate sleeping on the ground. If I didn't need ground to walk on or measure resistance to, I think I could happily give it up.