Hi
Apple does not offer the option to mix and match bands and cases when buying Apple Watch, whether in store or online. When initially buying Apple Watch, it is only available in certain preconfigured combinations of band and case.
For all preconfigured combinations of Apple Watch and band, the product description in the Apple Online Store states the wrist circumference that the watch and band combination will fit.
For example:
- Apple Watch - Space Gray Aluminum Case with Gray Sport Band - Apple
- Band with 42mm case fits 140–210mm wrists.
Stated wrist sizes relate to the wrist circumference that the combination of band + case is designed to fit.
When buying an Apple Watch with a Sport Band, the supplied Sport Band includes a component for configuring the band for a S/M (small / medium) fit and a M/L (medium / large) fit.
When buying an additional band for a 42mm Apple Watch as a separate accessory, the Black and White versions of the Sport Band are available in an M / L and L / XL size option, which fits wrist circumferences of 160–245mm (when worn attached to the Apple Watch):
Link Bracelet kits are also available for extending the size of the Link Bracelet and Space Black Link Bracelet.
The product details pages for all other bands also specify the wrist circumferences that they are designed to fit, when worn attached to the Apple Watch.