I used a 5 gallons SS pot for over a year, employing the BIAB (Brew in a Bag) method, while brewing AG. RM-MN superbly explained what BIAB is, but you can always do some research, plenty of resources online, if you finally decide to go this way; it is indeed a thing of beauty. I adjusted my recipes for 4 gallons, and since I bottled, that gave me between 35 and 40 bottles of beer, more than enough for my drinking needs. I also brewed 5 gallons occasionally; in these cases I simply calculated the recipe for the final volume, but considering the smaller boil volume for hop utilization, I believe all online calculator tools will differentiate between the batch size and the estimated boil size. In order to get the volume to the final 5 gallons, I simply added chilled water previously boiled to the final wort, or added water to the pot gradually while boiling. It's not ideal, but trust me, it works. The smaller batch sizes also allowed me to brew more variety, and made it easy to move on from beers that didn't come out too good. I now own a ravishing 10 gallons pot, but got so accustomed to the 4 gallons batches, that is I still what I brew.