As long as you have the capability to 'turn down' your SWCG, bigger is better than smaller. On the same pool, a 2x unit will last almost twice as long as 1x unit, but rarely costs twice as much. Even if the unit doesn't have sufficient native 'turn down', a time clock on the power supply will give you that control.

Two reasons NOT to get a bigger unit:
#1 - The SWCG is going to be really close to the pool, so the generated chemicals (gasses) don't have time to fully dissolve.
#2 - You have a poor distribution system and low circulation, so most of the chlorinated water will end up in one 'hot' spot, which could end up being the dead swim suit zone.