I know some people get aggravated when I say this, but I’ve put 2x4’s flat, on edge, and I’ve used tree branches just to see what they prefer. They tend to keep using the things they are used to if they are the same height, but if you mix them up so they have to make a decision, I’ve determined people care about this a lot more than the chickens themselves do.
Because of the length of their toes I don’t like to use anything too small a diameter so they can get a pretty good grip. But that’s my personal preference. I’m not sure how much the chickens agree.
Here are some very old shots of a tree branch as a roost. You may be able to get an idea of the thicknesses. See where they are roosting so you can see what they prefer. There is a window near the thicker end so I flopped this to see what they would do, putting the narrow side near the window. They pretty much stayed the same, with the chickens that preferred the window staying near the window. That cockerel was relatively young and the dominant hens preferred the window. He did not argue. But when he grew up, he moved close to the window.
GPeters it looks like that round roost may be a metal pipe. I don’t think it’s so slick they can’t get a grip, but how is it fixed? Did it rotate under the weight of the heavier birds and throw them off? That may be why your bantams could use it but your larger birds could not. That’s just a guess though, I don’t know.
I don’t know how cold your winters get, but I’d be concerned that a metal roost would be so cold that frostbite on the feet is a possibility. I really like a wooden roost. Regardless of shape, when mine squat down on the roost and fluff up in cold weather their feet disappear under the feathers. Wood is a good insulator. They’ll warm up the bit they are setting on and spend the night pretty cozy. My temperatures only get a few degrees below zero Fahrenheit most winters so I don’t get the bitter cold some people do, but here I’m not really worried about it.
As others have stated they tend to like to roost on the highest thing available, regardless of shape. They don’t always but they tend to. The ones higher up in the pecking order get first choice. If a lower ranking hen gets uppity and tries to take a preferred spot, the higher ranking can get downright unpleasant. This is most noticeable when I’m integrating younger birds, but even when they are all adults the dominant ones tend to sleep together and the lower ranking sleep together but separate.
My suggestion is to put in a roost, use whatever you want, up higher than the nests and situated however you want it. Give them plenty of room so they don’t have to crowd together. It looks like you have enough open space in the coop that they can spread their wings and fly up, though an intermediate step might be used. They are creatures of habit. If they are used to sleeping in your nests, they may continue doing that. If they do, after dark physically move them to the higher roost. Make sure the coop is dark enough that they can’t just hop back down. Or you might block the nests off so they can’t get back in, but make sure you are out there pretty early to open the nests back up. You don’t want to teach them to lay somewhere else. They might get the message of where to sleep after a time or two. They may be more stubborn, but if you are consistent they should get the message.
Good luck, this can be frustrating. The cause and answer is not always obvious.