Fermenters are available in a variety of sizes and formats to accommodate all consumer programs and residential development needs, such as large size fermenters. Size is the primary factor you need to consider when purchasing a fermenter. Throughout the fermentation process, in addition to the wort, a thick layer of foam, called Clausen foam, forms. If your fermenter is too small, the foam will definitely increase through the airlock, possibly clogging it and triggering a blowout.
Your fermenter must be at least 20% larger than the amount of beer you intend to produce. For a 20 liter (5 gallon) set of beer, you will need a fermenter with a capacity of at least 24 liters (6 gallons).
Once you have determined the required fermenter size, you can choose the most appropriate design and product. Each fermenter has its own drawbacks and advantages, but in the end, your choice is largely a matter of budget and personal choice.
Throughout the fermentation process, in addition to the wort, a thick layer of foam, called krausen foam. As fermentation nears the end, the amount of krausen gradually decreases. The krausen can only be discharged through the airlock, which may be blocked if you want to fill the fermentation vessel with wort. After that the pressure will definitely build up in the fermenter until the airlock is burned out.
To stop this, you must choose a fermenter that is 20% larger than the amount of beer you will produce. This will leave a gap at the top of the fermenter called the headspace.
So in order to make 5 gallons of beer, you would need a 6 gallon or 6 gallon and 50% gallon fermenter.
You can either bottle or keg your beer, or you can move it to a second vessel to age it after fermentation is complete. The latter therefore also requires additional headspace in order to develop the Clawson. The second fermenter must be as complete as possible. This reduces the amount of air that is exposed and the chance that the beer will oxidize and develop undesirable off-flavors. For a 5-gallon kit, you will need a 5-gallon second fermenter.