The more air stone the better, you might want to consider having 2x 12"... And your going to want to get an air pump that is designed for at least 4x the amount of water...
As far as is 5watts enough... possibly i think those typically for like 80gallon fish tanks... in that case you should be good. But if you ever get a larger settup or plan on increasing from just 1 bubble bucket you would want to consider getting a larger air pump and using the 5watt for a backup.