I've got a group of seven yo-yos and six clowns in a 100 gal and I'm not sure I can imagine them in anything smaller, even if I had only one group. They were purchased pretty small. Two to four inches for the yo-yos and a little over an inch a piece for the clowns. I'm surprized by how fast they increased their size. The yo-yos are incredibly active and rambunctious and all over the tank. The clowns are not as "wild" but use every inch of the tank. My intention is too eventually separate the group and house the clowns in a 200 gal.
From everything I've read clowns require a minimum of 75 gal.
Clowns potential size can easily reach 12 inches and on up. I've read they will get to 6 inches pretty quickly but slow down a little after that.