You are also the only person with the authority responsible for dealing with problems arising from the behaviour of individual students. The size and composition of groups will depend on the online activities you have planned. As the instructor, you might form the groups or you might ask the students to organize into groups themselves. If you are teaching students in a distributed mode and there are no opportunities for the group to meet, you will need to organize the groups because e-mail and conferencing systems don't yet provide the tools to automatically facilitate group formation. Any students remaining can be formed into a group.
Group size is an important variable for success.
Discussion groups of about 15-25 seem to work well. For more complex projects, groups of between two and four work well. It's a good idea to have one list or conference for announcements and separate lists or conferences for discussions and assignments. Online group work benefits from members taking responsibility for different roles. One role is the group coordinator whose job is to track completion of the activity and ensure that the work is proceeding smoothly and on time.