I love tarp camping, and in my mind a tarp is the ONLY shelter choice. I've been using one year round for the last 2-3 years and have zero desire to ever sleep in a tent again. I've slept on thick blocks of ice on the side of a glacier surrounded by snow up to my shoulders with just my tarp keeping the wind and snow off me (pegging it down is tricky in this situation.. lol). They work.
I
greatly prefer a square tarp. 9'x9' I think would be my sweet spot. Right now I use an 8'x10' because it is the most commonly available size, but a 10'x10' would be pretty easy to find as well. Anything in that region will very comfortably sleep two people, and can easily be configured for more. I always hunt, fish, and do general backpack-y things with my girlfriend, so carrying a simple tarp is the easiest way of sleeping two people. I typically like to bring one of those reusable emergency blanket / space tarp things as a ground sheet and fire reflector.
Here's my two person overnight set up. Everything seen fits inside my little Kelty MAP pack, minus my girlfriends sleeping roll and pillow -- I make her carry those.
Unrelated topic, but take THAT, popular youtube gear reviewer who says the Kelty MAP is "only" a day pack! I've done three days out of it, and have done countless overnighters... carrying shelter for
two. If I carried a light weight sleeping bag for one person (wool blankets are extremely bulky), and a smaller silnylon solo tarp, it would be even easier. But I digress.
Bottom line: Tarps are amazing if you don't mind being a little bit more "exposed" than a tent. I actually prefer it. No condensation problems, and I can see everything around me.