Hare, Borgatta and Bales (1966) claims that dyady should not be considered as groups:

since the secession of either participants would result in the destruction of every relationship, dyads do no constitute a social structure per se. On the contrary, groups of three and more people allow their member to have indirect relationships with one another, and can maintain a given organization in spite of personal changes.

-> Small groups are made up of 3 to 20 members