All bracelets are strung on high tensile rubber cord. This is to allow them to stretch over the widest part of the hand. They will endure stretching over time, as long as the rubber is not stretched too far in one use, in which case the rubber may not return to its original length.
To determine how the bracelet will fit: With a tape measure, or string, measure around your wrist. Add 1 3/8" to that measurement. For a snug fit bracelet, pick the size that is closest to this measurement. For a looser fit, pick next size up from the "snug fit" size.
For example, for a wrist that measures 6.5", adding 1 3/8" = 7 7/8". The snug fit size would be medium. The loose fit size would be large.
small: 6 7/8" - 7 3/8"
medium: 7 1/2" - 7 7/8"
large: 8" - 8 3/8"