What size of Pressure tank do I need?
Pressure tanks are sized in accordance with your pump specifications just as your pump is specified in accordance with the amount of water your Well makes. When sizing a tank, it is important to determine the ga/min your pump can provide and match that to the draw down capacity of the tank. The idea is to allow your pump to rest for a minute or two as the tank is emptied thereby limiting the wear and tear on your pump.
The most common pump size is 10ga/min. So in order to ensure your pump does not cycle too often, you would require a tank with a draw-down of approx 15 - 20 ga.