The size of your tank in this situation will not matter, it appears that your pump can't keep up, or, more likely, your well recovery rate is too slow. The tank is a buffer for the pump so, under normal conditions, it doesn't have to run continously. Depending on the size of the pump, it is preferable to let it rest a little bit inbetween cycles, but if it can't keep up with filling the tank, it won't have a chance of keeping up with an open hose to fill an entire swimming pool. That's a huge load on the pump. You might consider having a tanker bring in water to fill your pool. My unprofessional opinion.