Personally id get as big as you can. I think a 30" would be a bit small, thats less than 3 foot in length.
A 48 or 42 if u dont have enough space for biggest, would be ok as long as they get time out of the cage to run around too tho. I have a playpen attached to mine so they get more space when im at work or asleep. With a 42 or 48" one you would also have space to add a shelf if you wanted to. :wink: