If you are not sure of the Scroll Wheel Encoder size needed for your Mouse, it is best to measure the height of the current Encoder. Many Encoders will have a number stamped on the face plate. As an example a Scroll Wheel Encoder stamped with a 9 is 9mm. If there is no number stamped on the face plate then you will need to measure it yourself, this can be done with a metric ruler or dial calipers
To measure the height of your Scroll Wheel Encoder, you need to take a measurement from the base of the mounting plate to the center of the Encoder wheel holder.