Look up the installation instructions for your tank. Tank should be precharged to the same setting as your fill prv valve setting. This must be set before the tank is pressurized on the system.
The tank size is governed by the fill pressure setting, relief valve setting, system volume and system temperature rise. Too small a tank will not permit sufficient expansion and may cause the relief valve to open.
Adding air to a tank hooked to the system that is pressurized and warm is only a temporary fix.
I have 2 expansion tanks with shut off valves at each tank. The valves are the type that have a bleeder to relieve the pressure before checking /adding air to the bladder. I just make shure I open the valve when done and use a zip tie to keep it open (just so kids don't tamper with it). Remember pressure is higher at the lowest point on a water column. With 12 lbs of pressure at the bottom, if you were to measure around 24 feet up on the column, you would have 0 lbs, measure up 12 feet and you have 6 lbs of pressure.