The hides - and their openings - should be pretty small in relation to the snake. The snake should be able to comfortably fit inside, but it should be snug enough to "hug" him/her on all sides. Having a hide with a much lower ceiling will help too.

For reference, check out this thread on black box hides: http://ball-pythons.net/forums/showt...lack-box-hides
Although this thread is about a specific type of hide (sold by ReptileBasics...very affordable, I might add), it shows proper sizing of hides in relation to the snakes. You'll see the openings are just large enough to allow the snake to enter/exit.

Here's another useful reference for hide sizing: http://ball-pythons.net/forums/showt...0-Hides-my-way