I'd recommend buying 20gal (80qt). I went with a 60qt and boil overs are very common occurrence. To get 11gal into my carboys, I have to start the boil with ~13.5gal. Throw in a few ounces of whole hops and its dancing on the edge for the first few minutes of the boil. Fermcap is an option, but I just don't like to add non-essentials to my beer (call it my own personal beer purity rule).
Homer: Im feelin low, Apu. You got any of that beer that has candy floating in it, you know, Skittlebrau?
Apu: Such a product does not exist, sir! You must have dreamed it.
Homer: Oh. Well then just gimme a six-pack and a couple of bags of Skittles.