I have used a 5x8 poncho tarp for this purpose without too much trouble. I.E. for what you describe where it is usually easy to find a place out of the wind, sleeping under in mild weather etc.

Were I planning on tarping in really bad weather and wanted some comfort, take an 8x10. but you *can* do just fine in a 5x8 - it's just if the weather gets bad you don't have a lot of room and have to pitch it pretty low.

to put it in perspective, I take an 8x10 silnylon tarp hiking normally, that goes over my hammock, or I pitch on the ground if there are no trees. The nice part is I can be under it for days in pouring rain and it's like a palace.

But for what you describe (dayhikes, emergency, sleeping under on a superlight trip in probably good weather) I take a 5x8 integral designs silponcho.