IMO there's a lot to be said for having two openings (towards each end) or the whole front open, rather than just one central opening... prevents one hen from being able to monopolize the box by parking in front of the doorway and pecking all comers, and also greatly reduces the chances of someone getting cornered and beat up.
I have what I guess you might call a 'semi community box' in one pen, it's maybe 30" long (ish) and maybe 16" deep and high; it's where they usually prefer to lay, as opposed to the two normal individual-style nestboxes also accessible from that pen.
Good luck, have fun,
Pat