Traditionally, gloves are measured by the girth of your hand.
That is the circumference of your palm measured from underneath where your fingers begin.
Imperial glove sizing uses this measurement in inches. So if your hand measures 9" around, then you are an imperial 9 or a modern size L.
The length of your fingers is supposed to be proportionate to the width of your hand.

Sizes Small Medium Large XL 2XL
Imperial (inches)
7 8 9 10 11
Palm Circumference (cm) 17.5 20 23 25.5 30.5