In a 6x4 shed that is totally converted, you could probably get upto 25 medium birds, it would be full full, so no scope for massive change. There are probably paints out there designed for the job but will cost a bomb, ours is just varnished on the floor, and just stained on the walls (cheap and cheerfull).
The perches should be about an inch to an inch and a half in diameter, preferably rounded, like a branch, or curtain role, (again cheap, but works) the heights are really up to you, the hens will use whats provided, mine have 3 different height, one about 3foot from the roof, one below that and another about a foot above the ground.
Remember a trap door for them to get in and out (not essential, but lets les water in, if it rains)
Old shelves or drawrs work well for nest boxes (i'm a right skinflint me)
good luck,