There are a two main useful measurements you can obtain for mice, that directly tie into the size of a mouse.

Height Below the Index Knuckle, the tallest point below your hand.

Horizontal Grip Width, the minimum distance between the thumb, and typically ring fingertip.

Both are best correlated by a measurement of Hand Length, though there is variation in both, because there are more factors at play.

Prerequisites

If you wish to approximately measure Height and Grip Width. The details, for the purposes of getting measurements under the same conditions, are below:

• A rigid measuring implement with resolution to the nearest mm, that can measure from the desk vertically is a requirement (so, a ruler)
• You must have a good seating arrangement and posture
• Your elbow should be bent at approximately 90 degrees
• Most of your forearm should be on the desk
• The forearm and elbow must be level with the desk
• The base of your thumb must be on the desk
• Remove any particularly thick/voluminous or restrictive clothing
• Make a note of how much blank space is on your measuring implement
• Stretch, splay and flex your fingers and wrist a few times

The resulting resting position should look approximately like below.