Our nest box is 6" off the floor and roost is at 24" stretching the 4' across our coop (ours is 3'x4'). I think that setup will be fine for you though you have 2 more birds to contend with, so they may get a little crowded on there. If it's possible, I would maybe do two roosting bars 12" apart, one at about 20" high and the other at 24-26".I've found another thread about poop boards https://www.backyardchickens.com/t/621363/poop-board-convert-warning-graphic-gross-poop-pictures/20 and I will probably build one under the roost that I will install. I like the idea of sweet PDZ. Sounds like it lasts forever if you use a kitty litter scoop to pick out the poo. Also kills any smells that might be there too.
Question - My nest boxes will be external to the coop, 12 inches off the floor - How high should I make the roost? I'm thinking 24" since the front side of the coop is approx. 4 foot tall (outside dimension). I'll probably just do 1 roost the 4' width of the coop. Does that sound good?