Very few homebrewers go past 15 gallons. The ones I know that do 10 or 15 gallon batches generally use multiple 6.5 gallon carboys. Anything bigger than that and you can't move them around easily. Personally, I stick with 5 gallon batches because I like making different ales.
Once you cross the 20 gallon line, you are talking dedicated brewery space and lots of cash. A 27 gallon conical fermenter is $900. Full (20 gal.) we are talking 210 pounds. This large a batch will cook itself unless you have a cooling system, which is not part of the base price.
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