My 2 month olds don't roost. They continue to sleep in a pile for several more weeks. Doesn't matter which coop they are in, what size of roost, what height, or how many days you stick them on it until of course they get a bit older and start doing it on their own. I haven't had any start roosting that early yet. I also haven't had any fail to roost when they get older even if they didn't have a roost previously. I had some standards in a pen for 3 1/2 months, put them outside, and they followed the other chickens up on the roosts by day 2. I kept some bantams in a pen with no roost until months after they were laying and then gave them a roost. They were sleeping on it within days with no help. If your coop is getting that dirty from 8 week olds it's not going to hold up to adults. If you do get them roosting all of that mess is just going to be under the end of a roost and it's going to multiply 20 times as they get older.
My bantams like 2x4s. They can fit their whole bodies on them so you can't really get too big of roost since they'll just sit on them. They do make for a bit of a mess though because the chickens will poop on them. 2x2s or round roosts can be easier to clean but more likely to result in frostbit feet especially for larger birds. In the other extreme my seramas and a few other small bantams keep insisting on roosting on the edge of the chick pen made of some metal mesh panels that aren't even 1/2" diameter bars on the top. They have yet to fall off.