Crimson Trace makes two styles for N frames- square and round to square. Barrel length has no bearing on frame size. Here are also two compositions, hard and rubberized. I prefer the latter for CCW. I have them on a 4" 627 and 2-5/8" 629. Well made and not too sticky.
CT no longer makes round grips for N frames. This may change with S&W in charge.