Size #1 is recommended for children ages 5 to 8 years or adults using light casual shoes for walking. Also recommended for elder adults or some special needs situations. If the child is highly active or large, consider size #2. When in doubt, try a larger size.
Size #2 is recommended for ages 9 to adult for running, play, going to the gym, casual shoes, and active use. This is the best size for most adults including men who run. If you are a large adult, have heavy shoes, or want to use Zubits for recreational sports consider size #3.
Size #3 is recommended for large people over 190lbs (86kg), or anyone who wants to use Zubits for recreational sports. Size #3 has the most secure hold for active use. The difference with Size #2 is the advantage that it is a little easier to pop off when you want to remove your foot. Size #3 is also recommended for boots or heavier shoes with stiffer soles and people who like to tie laces really tight.