Everyone has a different sized jaw. Personally, I do not even use gags at all which have less than a 2" diameter. Recently, my husband bought me a gag with a 3" diameter. It all depends on preference, and how big your jaw is.
Also, I think that if you can spit out a gag, it's not tightened enough. Even smaller gags can be impossible to spit out if you tighten the straps enough.