The old adage is a good one: vessels should be twice the size of your batch.
60 qt (15 gal) BK = 7.5 gal batches.
80 qt (20 gal) BK = 10 gal batches.
I also use a 20.5 gallon (82 qt) Coleman cooler for mashing and batch sparging. Realistically, when mashing in, I never try to fill the cooler with more than half+1 gallon of the total water needed.