In groups of size three, there usually is enough going on to keep the conversation going long enough for the students to make progress on the more challenging problems. Sometimes two is too small, and five is definitely too large. Actually, I would have a hard time deciding between 3 or 4. It is useful to have one person assigned to keep the discussion going, another to keep a record of what has been concluded, and another to present the conclusions of the group. With three or four, the breakdown into couples does not occur.
Larger groups cause a breakdown.
Plus, even if they are on-task, a group leader will usually emerge and the weaker students in the group will have their questions answered in a sophistic fashion, rather than a clarifying fashion. I had the greatest success with groups of 3 students. With groups of 2, I found that students paired up socially or with one of much better ability. With 3's they seemed to even out more. Also, if there is trouble in the group I find that usually 2 people will come and talk with me, whereas a single person would be reluctant to talk with me.