I agree that the 21" is a good middle of the road baton, however, the 26" seems to be the most common for patrol officers. I have a 31" that I used to carry when working a private security job. Works great in crowd situations. The one time I had to use it I was grateful for the extra 5" because I was fighting with a guy carrying a bat.

My current carry baton is a ASP Agent LeverLoc in 16". I like the fact that is has a clip on it so I can carry it in my pocket instead of a belt holster.