It also depends how close to the hammock you rig the tarp bill.

I have kept a hammock dry under a 5x8 rectangle, but you have to be pretty tight up against it if you get a sidewind and have the sides of the tarp down pretty good.

A square tarp witht he diagonal dimension a little longer than the hammock is your best bet for "minimal" coverage, and you rig the tarp diagonally over the hammock.

HAving said that I *usually* use an 8x10 square. it's light enough in light materiel, and one of the nice things about the hammock is I end up with acres of space to stand around underneath, sit in the hammock sling style, watch the rainy world outside, invite nice young ladies over to chat and stay dry while amusingly watching their male companions (who decline the same invitaiton to wait out the thunderstorm under my tarp) set up a crappy huge dome tent in an inverted bowl of a tent site full of water with rain sheeting down..

For me it's those little things that make a bit bigger tarp nice - at least by the time I'm hammocking.