Hi
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.
The same information is also included within the product descriptions for all Apple Watch bands that are available to buy as separate accessories (measurements again apply to the band when worn in conjunction with the relevant size of Apple Watch - 38mm or 42mm):
You may prefer to visit an Apple Store or Apple Reseller to try on different Apple Watch bands before deciding which, if any, additional band to buy.