It is good to have different sizes of carboys. I have 6 gallon Italian carboys, which are closer to 6.5 gallons. I also have 6 gallon Mexican carboys, which are closer to 6 gallons. And I have 6 gallon 5, and 3 gallon BB's. (I also have various other 5 gallon, 3 gallon, and 1 gallon glass...