First you must find the flow rate of the pump. If you can find the model number it is usually possible to find the flow rate.
Another way is to measure it when the flow is steady. Connect a hose to an outside faucet near where your pump comes into the house. Turn off all the other water in the house and then turn on the hose. When the pump comes on, close down the faucet until the pressure (you do have a pressure gauge) remains steady somewhere within the range of the pressure switch.
Now measure how many seconds it takes to fill a 5 gallon bucket (5 gallons is usually about 2" from the top). The Gallons per Minute of your pump is 300 divided by the number of seconds to fill the bucket. (I'll bet you never thought you would need long division).
Pick a bladder tank that has a real volume of about 5 times the GPM of your pump. That will give you a cycle time of about 90 seconds with a 30 to 50 psi pressure switch setting. A smaller tank, 3.3 times your pump GPM, will give you about 1 minute cycle time. That is the smallest I would ever go.
Make the seller show you the REAL VOLUME, not the strange numbers that they quote for "galvanized tank equivalent".
When you install the tank, pressurize it with air to 2 psi less than the ON setting of your pressure switch. Make sure your tire gauge and your water pressure gauge give the same reading when there is water in the tank.