Nest boxes will work at any height. There is no one right answer for everyone. Some things to consider.
If you have them high enough above the floor and any litter that might be on the floor so they can comfortably get under there, that space does not reduce your effective coop size. I'd give them at least a foot above your litter if you don't put them on the floor.
If you have them low, you have to bend over to gather the eggs. How's your back?
You want your roosts higher than anything you do not want them roosting on, but you don't want your roosts too high. They can injure themselves jumping down in the morning, heavier breeds being more at risk. Also the higher the roost, the more horizontal room they need to fly down. If the roosts are too high, they can fly into the feeder, waterer, nest boxes, or walls when they fly down. They are not really very graceful flyers.
You want your roosts to be below your ventilation so they are not in a draft in the cold wet weather.
If the nest boxes are more than a couple of feet off the floor, it really helps to have a perch or ladder in front of the nest box to help them get in, sort of a step. Some people build a shelf.
The nest box itself needs to have enough height so you can put a 4" or 5" lip on it to keep them from scratching out the nesting material and eggs and still have an opening about 8" high for them to use to enter it. Depending on how you frame it, it may be OK as small as 12" in height or you may need 15" or 16".
That's the main things I can think of right now. Good luck!