As far as I know, fretboard radius on acoustics is either 12 or 16 inches. On electrics, it can be even more shorter. So you need a capo averaging the same radius as your fretboard. Believe it or not, no seller even bothered about that while selling a capo ! Why this is important ? If you use a 12 in. radius capo on a 16 in. radius fretboard, it will not put enough pressure on middle strings and too much on lateral strings... and middle strings will buzz. On the other side, 16 in. radius capo on 12 in. radius fretboard will put too much pressure on middle strings, bending them and getting out of tune. The capo, as well as fretting strings, should be just behind the frets with just enough pressure (too much pressure will aggravate fret use).
Classical guitars have larger flat fretboard, so another type of capo is needed.
I have at least three (maybe a fourth somewhere I do not use anymore) capos !