I use a forty quart white rectangular (marine) igloo cooler and do 5 gallon batches with it. It holds easily 25 pounds of grain with strike water, but if you want a thinner mash or do bigger beers a lot, the 70 can handle pretty much whatever you need and with batch sparging, you simply sparge a second or third time, if necessary, for a monster grain bill. If you are doing a lot of big 10 gallon batches, then consider a bigger one yet, I suppose, but then I go to my 3 keg system for that kinda batch.