I think its one of those personal choice things with no real method involved but what ever works best for yourself.
I spent ages looking at tips, changed my mind so many times before commiting to buying a set and finally ended up with ones i really like.
there is an absolute shed load of threads on here on the subject, maybe worth having a trawl through to see what options are open to you, and there are a fair few to be honest, many will may include changing over your rear bumper valance , well if changing tip style or to dual tips or even oval etcs.
maybe choose a tip style that you are happy with first then go throught the sizing options to fit your application.
rob