I love this video:
http://www.youtube.com/watch?v=7HiO9rzU_aw
That's how mental Nethies can be!
So BIG as possible! If outdoors and you're going for a hutch option, the minimum is a 6x2x2 hutch, with a permanently attached run at least 6x8!
Or a shed with a run, or a playhouse with a run. BIG basically!