The vizards wear hollow masks when they fight, other then that, they look pretty "normal."

Arrancar on the other hand, change completely. An arrancar is basically a hollow who has cast off his mask, showing a human appearance underneath. They aren't blackish...things anymore, but rather human-like. Bits of mask remain on various places of the body, and the hole where the chain of fate used to be is also present. Most are human size, but there are the over-sized ones (Yammy). They look like this all the time, not solely when fighting.

Minazuki is quite large. Large enough for Captain Unohana and her vice-captain to sit comfortably on top of her while she transports the wounded. I'm not quite sure precisely when you first see this, but it is sometime during the soul society arc, so it shouldn't be too far away.

Yeah, it would be kind of uncomfortable to sit in an animal's stomach to get healed.Espically since you get spit up when all's well....