There is no rule where to put the nest boxes. Many people put them om the floor. Others have them at many different heights. There are some examples on this thread.
Nesting Boxes
https://www.backyardchickens.com/forum/viewtopic.php?id=41108
You do want the roosts higher than the nest boxes. That makes a big difference. In the small coops you may not have much choice, but if you do, a few things I'd consider.
Unless the nest box is raised high enough that the chickens can get under them, they take away from available coop space. That height off the floor includes the height of the bedding. Also, expect the chickens to do a lot of scratching. Don't be surprised if the bedding piles up underneath the nest boxes, especially if you feed in the coop. They will scratch a lot more around the feeder than under the nest boxes, so the bedding can build up under the nest box.
If the nest box is on the floor level, they might get a little dirtier. The chickens might scratch stuff into them or they might play more in the nest boxes. Both might cause more poop in the nest. This is not nearly as bad as chickens sleeping in them instead of on the roosts though. And they MIGHT get dirtier, not that they absolutely will get dirtier.
Some people have bad backs. They put the nest boxes up where they don't have to bend over to get the eggs.
Most chickens don't seem to mind where they are. I'd suggest putting them where they are convenient for you.