I would say like a 2x4 or 2x5 with three levels. How often does she take them out? Because if they are free range during the day she wont need one quite as big.
Hmm- why have an animal if it's just stuck in a cage? Not to start anything- it just doesn't make sense to me...I wouldn't want to be cooped up all day. Plus- seems like alot of work with no benefits. I love petting and playing with my bun.