I have used tarps for about 30 years or so. It depends on how many people and for what purpose. A tarp is a basic roof. Chances are you will also need a bug shelter or good bug bivy, too. For two people, a 9'x11' works pretty well for me. I have had one for many years. For solo, a smaller 7'x9' tarp works pretty well. Actually I am sort'of looking for another. Mine is about 10 years old.
I prefer rectangular tarps only because thay are easily available. MYOG would mean a square tarp, much easier and a bit more flexible in setup. Usually a 11'x11' tarp mean a three sided pyramid about 4'-5' with a floppy door way.
I usually set it up as an A frame with a single pole and 5-6 stakes. A small 12-16" stick works OK for the rear, with a single hiking staff at the front. In the ADK's moisture, rain, mist, hail, etc is common. So, I pitch them low and wide. Tarps are generally for sleeping only, solo. With a partner, my most common methode is as a lean to. We sleep towards the back, stand up in the rain, generally gloat over our roof.