I remember someone suggesting years ago that the ideal crew size would be at least 3 people. The reasoning was that if a member was incapacitated, the odds of survival were a lot higher with at least 2 people to help instead of a single individual. Even in the reduced gravity of Mars, rescuing an injured person could be beyond the ability of a single individual.

To me, the real question is the set of skills necessary to perform a Mars mission rather than the number of people. At a minimum, the crew will need the following skills:

Pilot
Engineer
Flight Surgeon
Geologist
Microbiologist

and probably some others that escape me at the moment. Now, each individual will need multiple skills. For example, the pilot might also be the secondary engineer, while the primary engineer could be the backup pilot or even the Geologist. The Flight Surgeon could be trained to serve as the Microbiologist. Each primary position would need to be backed up by someone else on the crew. It'd suck if your only Pilot were killed in an accident.

Given this admittedly incomplete list of skills, it's likely that a crew of 3 or 4 would be adequate for a Mars mission.