I wouldn't go higher than 18" because of the small landing area available.
But if there is always 6 inches of bedding in the coop then a roost that is 24 inches from the floor will be 18 inches from the surface of the bedding. And if there is always 6 inches of bedding then a roost that is 18 inches from the floor will only have 12 inches of clearance under it.
Yes, the landing zone is small so you want the roost low. But if you make it too low then the birds won't use the space under it and it will be more likely that there will be birds crowding around in the landing zone when other birds are trying to get off the roost.