There is no substitute for a simple, flat (no curve in the center seam), rectangular tarp. It is the most versatile and pleasing of backcountry shelters.
But I’ll get ants in my pants! Don’t camp on an anthill, genius. If the skeeters are bad camp on high ridges, gravel bars in wide valleys, and little high points in the midst of big meadows (see below). If the bugs are really bad bring along a 3 ounces Nano.
But I’ll get wet when the rain blows! No kidding. Vary your pitch and most of the time, even with a smaller tarp, you’ll be fine. Sometimes you’ll get it wrong and have to get up in the rain and repitch, which sucks and will make you better. If the weather is going to be really bad make sure you can camp down in some trees or pack a mid instead.
Need further convincing? Joery wrote the definitive account back in 2011.
My flat tarp is 70 inches wide and 80 inches long, which isn’t quite long enough. I built it out of spinnaker fabric I got cheap, and had to run what I got. This summer I committed sacrilege and put a wall on one end, giving up versatility for a little more weather protection. For a tarp which will be a sleeping shelter for one and a cook tarp for 2-4, 6 to 7 feet wide and 9 to 10 feet long is a good size. Do not skimp on tieouts, especially a few in the middle of the ridgeline.
If I were to buy myself a tarp today, I’d get an Oware 1.8 flat tarp in silnylon. 12 ounces, 90 bucks, and 12 tieouts in all the right places. The well known ability of silnylon to sag over the course of a steady downpour is a drag, and the fact that my spinnaker tarp doesn’t do this is awesome, but I’d have a hard time paying 215 dollars (Zpacks) or more for a similar tarp in cuben, even if it is half the weight.
Used as a primary shelter a tarp will build skills and foster a neater, closer camping style. Used as a utility item a simple tarp is good as a tent vestibule, picnic shelter, sun shade, and drop cloth to keep bloody game bags off everything else in your trunk. Everyone should have one.