Fitbit Flex 2 Activity Tracker

Fitbit's slimmest wristband yet, the Flex 2 is a sleek, waterproof fitness tracker that lets you track activities, exercise and sleep. A removable tracker can be placed in a bag or pocket for discrete all-day tracking.
Specs

Small wristband: Fits wrists 5.5"-6.7" in circumference.
Large wristband: Fits wrists 6.7"-8.1" in circumference.
Tracker dimensions: 1.25"L x 0.35"W x 0.27"H.
Wristband width: 0.44".