I like to be able to hold my finishers. It's a personal preference of mine. I would tend towards a Coti that's about 6 or 7 inches long, by about 1 1/2 to 2 inches wide. Good for inside the hand.

Now, if i was going to do batch honing and I wanted to do it on a Coti....then I would want to cover as much ground as possible, like woodsmandave says. I'd go for an 8 by 3 then.