The bigger the tank, the less $/lb the propane will cost you as well (most places). So if you're buying, buy the biggest you can haul to the filling station.
And the larger the surface area of the fuel (larger diameter tank) the higher the potential evaporation rate (from liquid to gas), even when the liquid gets cold from evaporating. That's the reason for the pig tanks (laid on their side, tons of exposed area to evaporate).