I’ve just seen a piece on another forum, to the effect that the NYC PT Passenger Tender used with later Hudsons carried 18,000 gallons of water and 46 tons of coal. The UP Big Boy 4-8-8-4s towed 25 tons of coal and 25,000 gallons of water (all US measures, of course) with the Norfolk and Western Y class 2-8-8-2s in the same bracket. 

That seems to be about as big as it came, although some South African and Australian locos seem to have towed additional water cart tenders or tankers for maximum range (the NYC locos refilled from track troughs and refilled with coal part-way; the UP and N&W locos didn’t need the range, although that’s different in preservation)