Basically, as long as there's at least more floorspace than there is length of snake, it's deep enough for reasonable hides, and it's not so tall that it's difficult to keep the temps and humidity right, you should be fine. 3'x2' or 4'x18" or 3.5'x18" with a height of between 6" and 2.5' or any similar measurements should be good. If you know where you'll put it, measure your space and build accordingly.