I created a bit of a dilemma for myself. My one-gallon batch of acergyln (maple mead) got "stuck" at around 10% ABV, so I split it into two one-gallon batches and added more water, which got things going again -- but I needed to add back more honey and maple syrup to make up for the dilution, which I did gradually to avoid stopping the fermentation again. The only problem is, I must have added too much water, because I ran out of room in the carboys. I had two full all the way to the neck, and I still needed to add about a half-pint of syrup to each. So, I was forced(!) to go out and buy a 3 gallon carboy to re-combine the split batches in. It ended up being 2.5 gallons of liquid. So, anyway, happy ending. I now have (over) twice as much mead as I was planning for, and the 3-gallon size is much more reasonable (and affordable) for future batches than jumping all the way up to the 5.