CHOOSING YOUR CASE SIZE
Whether purchasing for yourself or as a gift, purchasing the correct size band for your Apple Watch will ensure the perfect fit out of the box. Each Apple Watch has a detailed engraving on the back of the case which includes the size of the Apple Watch Case.
In this example, this is a 38mm/40mm/41mm/42mm Series 3 Apple Watch. The larger version is a 42mm/44mm/45mm/46mm/49mm.
NOTE: Apple Watch Series 10 sizes include 42mm ( smaller version ) 46mm ( larger version ).