A square tarp (roughly 9'x9') is pretty hard to beat. Here in the ADK's that seems about the minimum for good coverage of you and your gear. It also sets up as a lean-to for those nights that it is not raining. Bug nets, bivy, or screen tent are pretty much required for summer nights, though. Spring and fall you can skip them. But, I have run into mosquitos in September.
An A frame seems to offer good protection. But I have also set up single poled shelters. These work well in storms with a 42" pole. These types of shelters are very dry on well chosen ground. The worst is water running in along one edge or another in heavy rains. With a larger tarp than 5x9, you can often shift stuff around a bit to avoid the worst of the water. But, I will agree, generally a square tarp has much better pitching options.