To determine your account's total estimated storage, do the following:
The screen that appears will have your user counts and storage. You should get a mail from us if you get close to your storage limit. Just remember, with revision control, a checkin is (almost always) forever, so don't delete files and check in the deleted version to try to save space; the old versions still have to be kept and so it won't help.
All storage numbers are estimates that we try to keep accurate to within a few hours. If you feel an estimate is way off base, just drop us a line and we'll investigate.