In an emergency setting up a tarp has value beyond the shelter from sun and weather it provides. Set up a tarp and people think of it as a landmark. I have noticed that even if you have nothing more than a box of bandaids with a tarp it is an aid station. A tarp set up high enough to walk under, a table and makeshift seating pretty much puts you in charge of the scene. Try it some time. Don't be surprised when people start coming up and asking where you want them to set up.

People will naturally congregate around any such landmark and assume anyone who looks like they know what is going on is in charge. This can be a double edged sword. If you don't wish to be looked to as an authority don't put up a shelter or, at the very least, use a tarp in a subdued color and keep it low.

Six foot square, roughly the size of a standard rain poncho, is IMO about the minimum useful size if you want a lot of options. Rigged low you can sleep under it and have complete coverage if you sleep on the diagonal.

Eight foot square or over opens a lot of options and allows the simplest, swiftest and most foolproof of all shelters, the burrito roll. Simple as: spread tarp, lay on one edge, roll over making wrap tighter at foot than head. With practice you get complete coverage with all edges tucked under you even in hurricane winds, decent ventilation and end up with enough extra coverage at the head to pull in and shelter your pack. Total time: less than a minute.

Helps if you rig a light line from the foot inside so you can seal or loosen the foot end as need be. The burrito roll is a last resort but one that has saved my bacon when I was wet, exhausted, at the edge of hypothermia and there was no shelter anywhere nearby. Once your warm you can climb out and build a more substantial and comfortable shelter. Or not.

Eight by ten allows you to use it as a lean-to and ground cloth in one.

Twelve by twelve is about all one person can effectively use in my opinion and is pretty much the point of diminishing returns if your not setting up a base camp.