This study aims to identify the optimal group size for collaborative argumentation using SNS for educational purposes. The study is based on the participation of high school students; the participants were divided under two experimental conditions. The students were required to engage in one of two types of arguments: argumentation via SNS or face-to-face argumentation. Furthermore, the students were divided into small groups comprising three to five participants for each condition. Conversations and task performances were collected as the students were engaged in discussions. The transition probabilities between two talkers in the group, the number of utterances, and the task performances were analyzed. The results showed that the 3-person group discussed in a lively manner; however, it was difficult to obtain the consensus of this group. The 4-person group did not converse much initially but eventually conversed enough to be compared with other groups. It was also easy to obtain the group's consensus. The 5-person group conversed less, possibly due to social loafing. The findings suggest that a 4-person group is the optimal size for collaborative argumentation using SNS for educational purposes.