My vote was on 6" or smaller. I find them most usefull on the 5"-6" blades. Those are often allrounders, and a choil adds to the allround-ness...

Also, on the choil-less knife in the picture, there will still be a good half inch from your finger to the edge. I like to place my finger right where the edge starts.
With all that said, the choil has to be thought through, and many aren't!

It all depends on what each individual like, and how each use their knife...