Even though the last post is already more than I year old I still find the topic interesting ^^
So at our station we have two Tankers, one with a 686 gal tank (the truck is a Mercedes-Benz form 1989 and will be replaced by a Rescue Engine next winter), the other Tanker is a brand new Rosenbauer AT3 with a 1320 gal tank, here's a pic of it:
We don't need anything bigger since we only run in the city, and even though the historic city center is close to 2000 years old and has a lot of the old structure, there's a very good water supply. There are at least two further Tankers in the city at the VFD, and the factory FD from the local BMW plant probably has one as well, same with the state's fire-fighting school in the city.
But I do know some FDs over here which run Tankers with 3500, 4028 or 6472 gallon Tankers: