Logically, the "largest" stock tank that could fit through the hatch and fill the space would be one or more bladder tanks. But how large do you need on an e29? Unless you are planning a major voyage, it may not be worth the hassle to keep tanked water potable. Most people I know take bottled water on weekend trips for drinking and just use the tank for wash water.
I haven't got to that part of the "refit" yet on my boat. A PO bought all new tanks, but the water and holding tanks don't actually quite fit, and are just stuffed loose in their lockers, with one corner sticking out. Will measure next time I'm in the boat (hiding from work.)