I use the MEC Scout silnylon tarp which is 2,1 meters by 2,9 meters (8' x 10', more or less). It's not cheap (64$ CAD) but I would never carry a cheap blue tarp in my backpack anyways (way too heavy and bulky).

The Scout (in it's silnylon version) is very light and easy to manipulate (it has a very soft, supple feel to it).

As for the rectangular/square thing, it's no biggie. I've used my tarp in several configurations (A-frame, lean-to). The secret here is to PRACTICE. I've rigged plenty of shelters using chairs and living room cabinet handles