Watch band or watch strap size is determined by the width of the band end that attaches to the watch. Watches come in many different sizes, therefore the width of the watch strap can vary as well. To determine the appropriate watch band size for your watch, measure it’s lug width.
Watch Band Measuring Guide
The size of the strap is a measure of the width of the lugs.
For example, if the width of the lugs is 20 mm, then your strap has a size of 20 mm
If you already have a leather band for the watch in question, you can check the back to see if it has a size imprint, shown in millimeters. Otherwise, you will need to find the lug width of your watch, which is the distance between the inside of the lugs on the watch head.