In about 20 years of fishing mainly on the Buffalo and Crooked Creek I've caught two over 20 inches with one at 20 1/2" and one at 21 1/2". I didn't weigh them, but I'm pretty sure they would've pushed 4 lbs. really hard and maybe gone over a touch. One float trip on the lower end of the Buffalo me and a friend of mine caught 8 fish over 18". I've also had some pretty reliable guys I know of tell me they've caught smallies between 5 1/2 and 6 1/2 lbs. on the lower end of the Buffalo. Personally, I believe them.
Last year was an excellent year on Crooked Creek. There were several trips when we caught around 20 fish between 16 and 20 inches. (See my post on the Crooked Creek forum for a few pics with one pushing 20") That is definitely not typical though, an average trip might produce 1 to 4 or so fish over 16 inches. All in all, I think anything over 16 is a good fish, but I'm happy to catch them just barely bigger than my bait as well.
To me the keys to catching the big ones are 1) High, stained water, (preferably in the spring or fall), or 2) Fishing at night in the summer.