The numbers for 6 or 8 chickens are very different.
8 chickens
8x4=32 sq feet for coop
8x10=80 sq feet for run
8x1=8 feet for roost bar and 8 sq feet for ventilation
6 chickens
6x4=24 sq feet for coop
6x10=60 sq feet for run
6x1= 6 feet for roost bar and 6 sq feet for ventilation
Frankly if you have the room I would build for 10 chickens then plan on having a smaller number. When I built mine I built it for two times the number I actually keep. That way as my birds age I can allow them to hatch out their replacements and take out the older birds as I want instead of having to take out the entire flock at once because of space limits. More room is never a problem, not enough room is.