There are many factors in planning a long space voyage. The human interaction being one of them. Group dynamics have been studied in detail for naval and other military applications, the stations on Antarctica have also contributed to the body of knowledge. As seen with Biosphere 2 the answers for space (or simulated space) trips are not as simple.
Logistical limitations put more restrictions on the number of members of space crew then common Military or Arctic journeys.
I have heard that the number of members of a team, can impact the relationship dynamics, where poor dynamics lead to mission failure. Presumably civilian vs military and/or gender of crew members would be considerations as well,
Has there been any significant research done to find the optimal configuration and number of members of a small (<= 5) team?