Should the groups be formal or informal?
An informal group is one in which the group is formed more or less spontaneously in order to quickly get to the task at hand, and which disbands after the task is completed (usually in a very brief time). For example, informal groups can formed by simply asking people to discuss a question with those around them. Formal groups, on the other hand, are created with a little more planning, the tasks are also more structured, and they tend to remain together for a longer time.
Should the groups be structured or unstructured?
In a structured group, each member is assigned a particular role to play (see later) or task to perform (or both) in achieving the overall group goal. In unstructured groups, the group is simply given the task to be performed and each person in the group is free to contribute as he or she wishes. Which option is chosen depends on the level of collaborative small group skills displayed by its members. If the group has people who have a high level of these skills, the group can be left unstructured. If however, the group is not functioning well together (say with some people dominating while others are left out or things not being done), then some structure might have to be imposed. But it should be always remembered that the goal of active learning is to make people independent and self-monitoring. Thus, in my opinion, structured groups should always be perceived as an interim measure and thus they should eventually learn how to perform well in an unstructured setting.
What should be the size of group?
Group sizes are determined by many factors. Sometimes it is defined by the number of groups that are required. i.e., If there are six specific and different things that the instructor wants to have done, then it might be necessary to divide the whole group into six small groups and assign each group one task. Or there might be a limited amount of equipment and other materials available. In these situations, the number of people in a group becomes determined by the class size and the number of groups allowed.
If such constraints do not exist, then the preferred number of people in a group is either three or four. There seems to be a slight preference for four in the cooperative learning literature but three is viable, especially for older people. Pairs (or dyads) are useful for specific situations (such as for quick discussion in bookends lectures) or when you want to ensure that everyone gets a chance to participate (in handling laboratory equipment, for example). In my experience, groups of sixor larger are to be avoided for any extended task. They quickly form sub-groups of two, three, or four. The larger groups also have the disadvantage that they enable shy, quiet people to withdraw unnoticed and not be part of the learning process.