We're much smaller than many others here... but just thought i'd include ours since we seem a little odd - given what we use.
SBS 2000 with approx 40 users.
priv info store is 13.8gb
largest mailboxes: about 5 with >1gb, maybe one or two with >2gb
retention: 15 or 30 day
GPO: outlook ask to delete items in recycle bin
obviously we have no quotas... and additionally we have a digital scanning device sending through email (approx 100 docs @ 80kbps/ea daily) which i've got set to auto delete after some time...
with our anti-spam throwing emails into recycle automatically, the GPO really helps people stay on top of deleting junk.
our backup is currently veritas to a tape drive, though i'm trying to look at using our point-to-point T1 to do some sort of sync (to a NAS) or VSS type solution. (somehow i'd like to combine VSS's history with block level sync'ing to the NAS... the backup would be off site, up to date, and have historic versioning :))
I realize we should upgrade to 2003, but our SIS ratio has historically been way to high to consider... the second we'd do that (aside from in-place upgrade or the unsupported forklift method) our info store would break the 16gb limit... hopefully w/ SP2 we can upgrade though! :)
my $.02
-Scott