This study examined the relationships among group size, participation, and learning performance factors when learning a programming language in a computer-supported collaborative learning (CSCL) context. An online forum was used as the CSCL environment for learning the Microsoft ASP.NET programming language. The collaborative-learning experiment was performed with one large group and 15 small groups. A total of 120 students participated in this experiment as part of a half-semester ASP.NET programming language course. The course contained an online forum for supporting the students' social activities and participation. This study used a participation-weighted rate for different participation types. A "learning score" and a "learning satisfaction" score were used to measure learning performance. The results of this study were as follows: (1) the online forum support aided collaborative learning, regardless of group size; (2) group sizes did not significantly influence learning scores directly but significantly influenced participation, and small groups had higher participation rates, which positively influenced learning scores; and (3) learning satisfaction using the online forum was higher than the average score. Small groups had higher learning satisfaction rates, and participation did not significantly influence learning satisfaction. Due to this study's results, we recommend that programs design instruction with small groups for teaching programming languages in online forums, support student-centered discussions, and encourage high levels of student participation to increase learning performance. (Contains 2 figures and 4 tables.)