I've bee wondering about that as well.
I've got an average sized cock. Above average sometimes, depending on the circumstances. :P
but I'm a grower not a show-er. The cock cage we've got now is a cheap one, made from leather and steel sort of like the one seen here:
I have a problem with slipping out of it. unfortunately, the cage doesn't hang like my penis normally does when it is flaccid. The cage is always either to the far left or right, or faces downward and presses against me. Is there a way to know how a CB is going to fit on you?