Interior nestboxes don't take away any space from the chickens if you have them mounted up on the walls like most people do, with space for the chickens to walk around below.
If a person has feeder and waterer indoors, the space they take up (which can vary from a lot to only a little, depending what size they are and whether they're in the middle of the room or mounted to a wall) should probably be subtracted from 'usable floorspace' calculations for SMALL coops -- but for a BIG coop, they really aren't going to make any much meaningful difference. Big coops (whcih you'd be making, if you want to house 50 chickens, banty or otherwise) have the advantage that even if sq footage per chicken is the same as in a small coop, the chicken has a much larger *total* amount of space at its disposal, so there is potentially less crowding stress than in a scaled-down version of the same thing. So, like 3 sq ft per chicken with 30 chickens in a 12x22 coop is a more generous arrangement than 3 sq ft per chicken for 2 chickens in a 2x3 coop, you know?
Good luck, have fun,
Pat