Due to the fact that George Zimmerman is being tried by a 6-person jury in Florida, there has been a renewed interest in jury size so here is a brief background sketch on the topic. Since 1970, the U.S. Supreme Court has recognized juries composed of less than 12 jurors (Williams v. Florida). In Williams, the Court determined that while the Common Law might call for 12 jurors "there is absolutely no indication in the intent of the Framers' of an explicit decision to equate the constitutional and common-law characteristics of the jury." The Supreme Court has gone so far as to say that 6-person juries are okay; however, the jurors must be unanimous (Burch v Louisiana).
Despite the Supreme Court's acceptance of 6-person juries, there have been numerous studies that overwhelmingly show that larger juries perform better. Here is just a brief sampling of what those studies discovered with respect to reducing jury size: (1) jury less representative of the community as a whole; (2) fewer jurors to remember important pieces of evidence; (3) the jury is less likely to overcome group biases; and (4) decreased accuracy in verdicts. While I don't think the aforementioned issues will necessarily occur by reducing the jury from 12 to 11, there is the potential for problems with 6-person juries.
For related posts on 6-person juries go here.