I use the Golite Cave 2 for the two of us. 10x12 feet according to a review, with small beaks, about 1' long.
Two people use depends on which two people, it's fine for my wife and I, but with a friend width is just barely ok in poor weather.
I find that this tarp, with small beaks is just long enough. Sleeping bag is 7.5', then some room for my head above it, means the minimum length to keep dry is about 8', but a bit longer is nice in case you slide around at night. Add about 1.5' at the foot end pitched low and about 2.5' at the head end since it will be pitched higher. This is about as short as I would go with a beaked tarp, so a bit more for a flat tarp.
How do the above people using a 10' long tarp keep their feet and head dry in wind blown rain?
You could figure out the width you need fairly easy:
Assume that in bad weather you will pitch the sides nearly to the ground. Take the width of the sleeping area you want, add some width to avoid touching the tarp, choose the minimum height you want and there you go.
3'/person+1.5' space, 3' peak height at head; gives root of (4.5x4.5+3x3) , is almost 5.5' width per side, so a 11' wide tarp.