The Apple Watch Sport Band is made from a flexible rubber material called fluoroelastomer with a metal pin closure. Sizes range from S/M, M/L, or L/XL. When purchasing an Apple Watch Sport Band, the S/M and M/L sizes are included in the box, allowing the band to fit wrists 140–210 mm.
Apple describes the Apple Watch Sport band:
“Made from a custom high-performance fluoroelastomer, the Sport Band is durable and strong, yet surprisingly soft. The smooth, dense material drapes elegantly across your wrist and feels comfortable next to your skin. An innovative pin-and-tuck closure ensures a clean fit.”
The Apple Watch Sport band has been available in the most colors so far. As of November 2021, 110 Apple Sport Band colors were available with an additional 36 Nike Sport Band colors (according to the Bandbreite app).
This example comes with S/M and M/L sizes for the 38 mm Apple Watch that is also compatible with 40mm and 41mm sizes.
Source: Apple, Bandbreite app