Our coop is 4'x6', and we've got 7 birds in it. I certainly wouldn't put any more in there, but they seem perfectly happy and healthy. They spend all of their time outside, though, aside from nights, and winter, of course.
As for roosting bars, I put mine up about 18"-20" inches, and put it right next to a large window, so they sit there all the time looking out, waiting to see us coming with treats, and then the squawking starts!
The unexpected thing that happened on the very first night was that they used the roosting bar as a ladder, jumping from the bar up onto the rafter boards of the coop. They spend each night up there.....seems to be their 'happy place' at night. If your design allows, I'd strongly suggest putting up as many roosting bars as possible right up near the ceiling.