I've got a 14" and a 20" The bigger ones are great because they're obviously quicker if you do alot of bigger areas.
I was doing alot of curved ceilings not long ago and found the bigger ones ideal for the curves because they're more flexible with the foam on, so it just glides around the curves easier than with the 14"
I think 14" and 20" is enough I wouldn't go bigger, but it's up to you.
They are worth every penny though, they definitely speed things up.