If I'm unlikely to have any really big girls (unless I adopt a full grown girly), it sounds like a 30-40qt would suffice. As far as tub dimensions go, I'm assuming that length and width have priority over height.

And, discovering how heavy melamine is, I may have to put off building the rack until I move, since I would have to ship it across the country. Though I have been entertaining myself by trying to plan out a few designs. My favourite so far is a small rack/bookshelf/storage design, with room for five tubs stacked length-wise on one side, and a few bookshelves (that could be converted to house smaller tubs) on the other side with a storage area underneath. I saw a design up in one of the stickied diy threads where a person had a mantelpiece like top, which I loved. I do like the idea of a rack with storage though.