Here is my layer "buckets" they are about 18" off the ground, there have been no issues with the hens getting into them but my Cochin cant fit so I have an old drawer filled with straw on the floor for her.
For standard size chickens... the "rule" is nest boxes about 16 inches off the ground. Roost need to be higher than the nest boxes because chickens like to be as high as possible to feel safe. And if the nest boxes is higher than the roost, you'll find them in the nest boxes. I have staggered roosts, one 3 ft off the ground, then 4 ft and at 5 ft. About 18 inches apart so they don't poop on each other as they roost! lol I do have a couple of bantam with the standard hens so I do have a ladder leading up to the 3 ft roost. The bigger hens like using the ladder also since getting up to roost is a little harder than their leaner coopmates.