My take on roost height is to make the roosts higher than anything you don’t want them sleeping in but as low a practical. Chickens tend to sleep in the highest place they can get to and they poop a lot at night. You don’t want that poop in your nest box for example. So make sure the roosts are clearly higher than anything else.
There are different reasons to not go extreme in making them too high. It’s unlikely but possible a chicken can injure itself jumping down from the roosts. My full sized birds spread their wings and fly down when they leave the roosts. Centrarchid made a good point in that different chickens have different flying abilities. Age, size, breed and such makes a big difference, but a few days ago my 8 to 9 pound full size mature rooster launched from a 5’ high roost, flew forward about 8 feet, took a sharp 90 degree left through the people door, and landed out in the run. I’ve seen other younger and smaller chickens do that a lot. Many people would be surprised at the flying ability of chickens when they are motivated to fly. I don’t have flightless silkies and I don’t have those huge Jersey Giants, just regular full sized fowl. I’m not in the least bit worried about mine hitting the ground hard enough to hurt themselves. Mine spread their wings and fly down. You are dealing with living animals and anything can happen but with mine the frequency is so rare (like so far, never) I don’t worry about it. There is another concern I’ll get to.
The higher you go with the roosts, the more coop space you need. Ladder roosts can take up a fair amount of coop space if you use those. If your roosts are real high you might need some intermediate perch to help some get up to them. My roosts are 5’ high and all the same level. My full sized chickens have no trouble jumping/flying straight up there, but they have plenty of room to spread their wings and fly. If your coop is so small they can’t spread their wings and fly, it becomes an issue.
Most of mine hop on top of the nest boxes which are a little over 2 feet high and then hop/fly over to the roosts when they are going up. When they come off the roosts in the morning a few might use the top of the nest boxes but most just fly straight down to the ground.
This is my real concern with coop size. The higher they roosts the more horizontal distance they need to safely fly down. They don’t just hop off and plummet straight down like a rock. They spread their wings and fly. If space is tight, they can hit nest boxes, feeders, walls, or anything else in there and hurt themselves. They need a clear space to land. The higher the roosts the more clear space they need.
Plenty of people have chickens that roost a lot higher than my 5 foot high roosts and their chickens don’t have any problem getting up there and don’t hurt themselves getting down. But their coops may be bigger than yours and set up differently. We all have unique conditions. I don’t see higher roosts as a huge serious problem, but I think keeping them as low as you reasonably can and still make them clearly higher than anything else is a prudent reasonable precaution.