With a 2-team playoff (as we currently have), you have #3 and sometimes #4 claiming they were unfairly excluded from a shot at the title. There have been years when there have been 3-4 undefeated teams, for example, but major conference BCS bias kept out folks like Boise St.
With a 4-team playoff, you may have some cases where #5 whines about not getting a title shot, but they really have far less claim when you consider that under the current system (or even the pre-BCS system), it's fairly rare for #4 to have a legitimate claim that they should have been considered. No, at this point, it's about schools wanting a shot at the money more so than the title. Of course a lower-ranked team can always beat a higher-ranked one, so the lure of a title is still there, but you know #5 is going to have to do it as an underdog.
As the field size expands, the situation continues in this same vein. #9 complains, but they're an even longer shot. Ditto for #17. For them, it's about qualifying for the lucrative and high-prestige tournament, not because they have any real hope of winning it. (Q.v., the NCAA men's basketball tournament -- starting around the #13 seed or so in each bracket, you're dealing with schools who are just happy to make the tournament. A #13 or #14 might surprise enough to make the second weekend, but no one lower than a #12 has ever made a serious run at the title.)
Bearing all of that in mind, a 4-team or 8-team makes the most sense. After you get past 4 teams, you're starting to really increase the chance for a team with a "failed" regular season to have a chance to win the championship, but an 8-team playoff would allow someone who just had one off week or some really bad luck a chance at redemption. 16 would be overkill.
(For the record, I think the NFL currently allows too many teams in its playoffs. As much as it would have been to my own team's detriment this year, I despise wild card berths -- they were a necessity when there were three divisions per conference, but it's grown into a monstrosity.)
Probably so, with maybe a wildcard week to give some of the weaker conferences a chance to qualify a team or two. But I doubt it'll go beyond four this year, and there is a meeting to take place today to discuss these and other NCAA matters,