Watch size band

Are you looking for the length of the band which is printed on the back of the band itself?

Or the width of the band which is related to the size of the watch case, and as such is printed on the watch itself.

https://store.storeimages.cdn-apple.com/4982/store.apple.com/shop/Catalog/US/Images/bxxd/size-guide.pdf

Look at the back of the band. Not all bands have the size printed on them.

If its not there, just measure your wrist and go by this table: