I have a row of nesting boxes about 30 inches off the floor of the coop. There is no ramp or anything like that, they just hop right on up there. The boxes are about 14x14 inches. Solar power door opens at dawn and closes at dusk.

Mine are about 2 foot off the ground with a ramp leading to them, they're about 18" wide by 12" deep. The chickens won't really care about height as long as it's easy to get to.