30 liters is a good gallon of drinking water. A gallon of water weighs roughly four pounds so the same weight of water in a tank would be approximately thirty pounds. A standard two-gallon carton or larger is not really designed for taking water directly from the faucet. A home faucet usually has a restriction on how much water can be poured out at once. In most homes, the restriction is around a quart. Since most households do not need or want to use 30 liters of water at any one time, there is no reason to use a larger bottle than needed.
When using bottled water for drinking or household cleaning purposes, it is always best to use cold water as opposed to hot. Hot water expands as it goes through the tap and expands as it sits in storage. This means that if you are using bottled water and it is already chilled by the time you fill your jar you will lose valuable water. To solve this problem, you should purchase a tank that is chilled before you fill any jar. Once your water is at the proper water temperature when you add it to any jar, you will have a much easier time transferring it to a drinking bottle without damaging it.
If you are a regular user of water from a tap, you probably only mix one full bottle at a time. There is no reason to use more than one bag at a time unless you are making large amounts of water. It can be difficult to make large quantities of water at home, especially if your water source is not a continuous stream. You can buy small jugs that are specifically made for mixing water. A two-gallon plastic jug or a one-gallon plastic drinking bottle can be used as a mixing tank.