I'm so glad you are preparing for this in advance. There's some calculation for size, but I don't remember it. I think it's 1 1/2 times her length (including tail) in both directions, but might well be wrong. It needs to be pretty deep. I've seen people say everything from 8 to 12 inches. I would say it's better to err on the side of bigger rather than smaller.
From what I understand, 5 months would be young, but when it happens depends on a lot of things and I've seen people say their 6 month old was gravid, so, like I said, best to be prepared!