Originally Posted by Rbeckett
Get the largest one you can buy. If you use the rule of thumb of 1.5 qts per Lb and the average grain bill is around 10-15 Lbs then the minimum to allow for boil off should be about 16 gallons to make a 15 gallon batch. Boil off will be something you have to kind of guess at till you get some experience with you specific equipment. If you can get a bigger pot then that would be better since you strike water can be sitting in the mash tun while your heating youre sparge water and you wont be delayed because of witing for water to reach the right temp. Once you boil you will lose a bit thus the minimum 16 gallon figure. 20 is better if you can afford it to minimize boil over issues and wort loss. Just my .02 and I'm still a newby but have been studying all the stuff to build a sculpture for all grain here. And for the record Stainless is the way to go....but the prices really hurt. Bigger is better, no matter what. you can always partially fill a container, but it will never hold more than it size allows.....
Anything much smaller than 20 doesnt leave room for a) boils longer than 60 minutes, b) prevention of boil overs, so 25 is really a more reasonable number.
Also: The stainless aluminum issue has been discussed to death, realistically the way to go is what you can afford/gets the job done.
Additionally, Really you need no more volume in your hot liquor tank than your desired final boil volume, as you can heat your sparge water during the mash if you dont use a direct fire mash system.