I've always thought a time limit on saved messages would be a better idea than a space limit. Well, I suppose you'd have to do a transfer-per-day limit too, to avoid being a pr0n/warez clearinghouse. But I know I've got stuff on my Yahoo account that I haven't seen in months just because I don't have the time to sort through it all. If I haven't saved it after two months, odds are I don't really need it.
So if you go ahead with it, I'd set a time limit on storage.