Adjustable snap bands can be used when you are not sure of the size of the wrist.
This type will come with 2 to 3 plastic buttons so that you can adjust wearing the wristband as per your wrist size.
These kinds of wristbands combine the stretch ability of silicone so it can be adjustable.
These adjustable snap wristbands are popular for promotional gift. http://www.amazingwristbands.com/Adjustable-Snap-Bracelets