different people have different opinions on this matter, the 10-4 formula is probably what you are going to hear the most and to be honest, it is a good place to start but is not always the best bet, look at how you birds react, are they fighting are they picking each others feathers or anything of that nature, if they are you have to many in the cage, if not they are fine. There are many things to consider such as breed, size of the birds, there personalities, how well they take confinement, distractions provided by you, age of your birds, how much time they will be free ranging, all these things affect how your birds will react to the size of there coop.
You are actually over the 10-4 formula now but I wouldnt worry, I would watch them and see how they react to each other before I would make any adjustments, as for the roost, higher is better just make sure they can all get up there, there nature is to roost in trees, so don't be to fussy about it they will roost 4" off the floor or at the top of the coop as long as there is room for them up there, they always go to the highest place they can in a coop.
Good luck