10 gallon for a pair (18"x12"x12"). They don't do very well by themselves, they just hide, get really stressed and die pretty quickly. Although they're not always the most sociable of fish, they need to have other fish around as otherwise they think everyone else in nearby shells is hiding due to a predator looming somewhere.
Although everyone says shellies do fine in small tanks, mine were much better when I had two pairs by themselves in a 29gal tank (upgraded from a 10) and now they're in my 4.5 footer with other fish, they happily patrol up and down the whole length of the tank (the two brevis males have basically divided the tank down the middle and have claimed half each, not just the 6" radius territory it says in all the books hehe).