I think you'll need to provide a little more information, such as:
- Is wind avoidance your primary concern?
- Worried primarily about precipitation?
- Do you sleep with a bivy?
- One person or two?
My first trip out with a tarp I used an 8'x10' tarp for myself. I was transitioning from a tent to a tarp and was a little concerned that I wouldn't like it. Here's my pitch:
And another view:
As you can see it's really only open on the end by the head and even those sides could be lowered even more if you were expecting swirling winds. The foot end was pitched in the direction from which the wind was blowing.