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.
If you wish to approximately measure Height and Grip Width. The details, for the purposes of getting measurements under the same conditions, are below:
(so, a ruler)
The resulting resting position should look approximately like below.