This study aims to identify the optimal group size for high school students' 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-toface 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 groups held discussions in a lively manner; however, obtaining the consensus in these groups were difficult. The 4-person groups did not converse much initially but eventually conversed enough to be compared with the other groups. It was also easy to obtain the group's consensus. The 5-person groups conversed the least, possibly because of social loafing. The findings suggested that a 4-person group is the optimal size for collaborative argumentation using SNS for educational purposes.

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