You need to check the flow while watching the pressure gauge to determine the flow rate while the pressure is steady (no water being added to the tanks).
When you are satisfied that you know the pump rate, I would get a tank with a real volume about 5 times the rate of the pump. That would be about a 40 gallon tank. With pressure switch settings of 30 to 50 psi it will take about 100 seconds of pumping to fill the tank. That is a reasonable cycle and will give you longer cycle times when you are irrigating.
If you are going to irrigate with your well, you need to have some idea about the well capacity. One inch of water on 10,000 square feet = about 6200 gallons.