The Nylon Loop band comes in three distinct sizes: small, medium/large and extra large. Each size is fully adjustable.

Small
Fits wrists 4.5 in - 5.75 in (114mm - 146mm)

Medium/Large 
Fits wrists 5.5 in - 7.75 in (139mm - 196mm)

Extra Large
Fits wrists 7.25 in - 9.25 in (184mm - 234mm)

How to Measure
Grab a piece of string (or anything string-ish). Wrap it around your wrist where you'd want the wrist band to sit. Mark the string where it overlaps. Then measure it with a ruler. Voilà - you've found the perfect wrist band size.