For a full-on Grand trip (18-21 days) you'll need food, beer, camp gear, etc, which is a pretty big heap of stuff. If you're running with a bunch of sweetheart big-boat mates who are willing to haul your weight (and rescue as necessary) then go for it in the wee bucket.
But be prepared to do your share and bit more in camp work, and thank them regularly. You might even want to shout some extra beer for the trip. Rowing an 18-footer heavily loaded is no picnic. That is, somebody has to carry your freight, and you'd best have an explicit understanding about that before starting the trip.