Where are they sleeping now? How old are they? Do you have other chickens in with them? What is some of the backstory so we know what you are dealing with? Photos of the inside of the coop might be useful.
My chicks may play on the roosts during the day but often don't start sleeping n them at night until they are 10 to 12 weeks old. Some wait even longer. That's why I'm asking how old they are.
Chickens tend to like to sleep on the highest location available once they decide to roost at night. That's why I'd like to know what they are doing now.
The way I generally determine roost height is to figure out where the floor is, including any bedding. Then install the nests. Some people like the nests on the coop floor, some like them high enough so they don't have to bend over to gather eggs. Then make the roosts noticeably higher than the nests or any other place you do not want them sleeping. If they are right at the nests or in a tiny coop 6" may be noticeably higher. Further away and 12" should work.
I like the roosts as low as you can reasonably put them considering all the above. The higher they are the more clear space you need on the coop floor for a landing space. You don't want them flying into a wall, the nests, or maybe a feeder or waterer on the way down. If they are flying up or down they need enough room to spread their wings. Sometimes access to the roosts can be an issue.
One easy way to catch chickens if you want to inspect them or treat them is to pick them off the roosts at night when it is dark. The roosts need to be positioned where you can do that. If yours is a walk-in coop that probably means waste to shoulder height. If you have a tiny elevated reach-in coop that depends on what it looks like.
Even fairly young Orpingtons can fly pretty well. If they want to get up there they can if they have room to spread their wings. Canoe mentioned 7', that sounds reasonable to me as to flying ability. Not a convenient height to pick them off the roosts.
I don't know what is going on. The more info you can give us the more likely we can make suggestions that fit your situation.