Here's my 2c worth; I use my 5 gallon SS kettle for the mash before the boil. I've mashed up to some 8lbs in 2 1/4-2 1/2 gallons water in it. So with 6 1/2 gallons of space in the cooler, I'd say you could do as much as 9lbs of grains in some 3 gallons of water in it, just off the top of my head. This pb/pm biab thing is getting interesting, since the more grains I use in the mash, the less extract I need to add @ flame out.