The most surefire way to get sizing right is to take measurements. If the thimble is a regular height option, you only need to measure from the tip to the base of the nail to know if it fits.
Longer thimbles may require you to measure between your first knuckle and the base of your nail. You’ll find that most thimbles will have measurements in terms of millimeters listed and you can thus countercheck that against your finger measurements.
Open, adjustable thimbles may cater to different finger sizes better if you can’t find listed measurements.