tailcaps have two dimensions that are important, the diameter of the body (not the shelf at the base), and the overall height. The height is less important somewhat than the width. You can measure your existing one with a simple mm ruler or measuring tape.
Most of these caps come with a 'nub' in the inside, which allows you to trim to adjust the reach between the button and the top of the actual switch. This can also be trimmed to adjust the sensitivity of how easy it is to turn the torch on.