Sizing a water pressure tank properly keeps the amount of run time to the water pump held to a minimum. The more a pump must run, the more wear and tear occurs to the pumping mechanism and the electric motor. An under-sized pressure tank can cause a water pump to run an excessively. In general, there is no problem with using a larger pressure tank than is required. All that is needed is adequate space for the placement of the tank.
Understand that most pressure tanks are rated by the draw down capacity. Draw down capacity is the amount of water that is stored within the tank before the pressure is low enough to turn the pump back on. Most private home well systems operate between 30 pounds per square inch (PSI) and 50 PSI. This means that the pump shuts off at the higher 50 PSI and turns back on to fill the pressure tank at 30 PSI.
Find the water pump's gallon per minute (GPM) capacity as stated in the manufacturers specifications. As an example a 1 horsepower (hp) pump is rated for 20 GPM, but is only able to deliver 10 GPM of water from the well source. You can find your own pumps capacity without the specifications.
Connect the garden hose to an outdoor spigot close to the well. Place the end of the hose in the five gallon bucket. Open the spigot valve wide open. Record the amount of time it takes to fill the bucket. If the bucket fills in one minute than the well pump delivers 5 GPM of water. If it takes two minutes to fill the bucket then the well provides 2.5 GPM.
Follow a general rule of thumb: size the tanks draw down capacity for every gallon per minute delivered by the pump. Using the above example of the pump's ability to deliver 10 GPM of water, the tank must be minimally sized for a 10 gallon draw down capacity.
Size the tank to a larger draw down capacity to keep the pump from running continuously if the household uses a lot of water. Place larger tanks in areas that have a solid floor such as concrete. Large pressure tanks can hold in excess of 200 gallons and can weigh upwards of 1600 pounds.