As I think about group size for RPG sessions (and working on the assumption of one PC per player) the following question comes up for me:
Does the ruleset impact on the group size?
I ask this, because most D&D styled gaming is based on covering the four main classes grouping: fighter, cleric, magic user and thief. Therefore, you need four players (absent multi-classing) to cover the spread.
Crypts & Things is one of the few OSR games that I can think of that does away with the spread of "core four". Fighting classes, an universal caster (mixing magic user and cleric) and a thief. But the thief isn't "needed". He's better at thieving than the other clases, but all can attempt the skills and all get better with them. In Crypts & Things, you can cover the spread with two PCs.
RQ, Legend, Basic Roleplaying, Openquest - you don't have classes, you have skills that define the character. With rules like this (and Savage Worlds and the like) you can create characters that cover more of the spread, if you will. They might not be specialized, but they should be competent.
I'm by no means saying you can't play D&D or a class based system with less than four players / PCs (or whatever the number may be). I ran a game AD&D 2e game for a year with just three players, and a Space Master game for nearly just as long with just two. What I am saying is that certain rulesets default to different group sizes by their very nature. They have a minimum number that fits their "sweet zone".
How hard do you find it if you fall below that number (no cleric or no thief in the party, etc)?