My opinion is: Excluding mini chain: 45cc is good for a max of about 16", 50cc is about right for 18", it takes 50 to55cc to run 20", may up to 65 cc to run 20 " at pro levels, you need about 69cc or more to run 24", 85cc handles 28". I know no one around here that is running more than 28" (I mean regularly)
Now I'm talkin central Illinois hardwood where there is hickory and hard maple.