There’s a six-person jury in the Zimmerman case — why six rather than twelve?
The Supreme Court has held, in Williams v. Florida (1970), that the right to trial by jury doesn’t require the traditional jury of twelve people; six suffices. (Ballew v. Georgia (1978) held that juries of five or fewer are unconstitutional.)
Nonetheless, while many states allow six-person juries in misdemeanor cases, very few allow them for very serious felonies, such as murder, for which Zimmerman is being tried. According to Bureau of Justice Statistics, State Court Organization 2004, tbl. 42 (supplemented by this statute), only Connecticut and Florida provide for a six-person jury for very serious but noncapital felonies, and only Florida provides for a six-person jury when the offenses is punishable by life imprisonment (with or without parole); Utah apparently provides for an eight-person jury; all other states provide for twelve-person juries. (Again, I’m speaking here of very serious felonies; a few more states provide for six-person juries for less serious felonies as well as misdemeanors.)
Naturally, having a six-person jury rather than a twelve-person jury makes it more likely that the jury will have an unusual demographic mix, such as the Zimmerman jury, which is all female and has no blacks (it apparently consists of five white women and one Hispanic woman). Having a six-person jury will also tend to decrease the likelihood of a hung jury, since a jury of twelve is mathematically more likely to have one outlier holdout than a jury of six (though of course there are complicated social dynamics of the jury in play, as well as the purely mathematical probabilities).
UPDATE: Thanks to commenter Gideon for noting that Connecticut provides for a twelve-person jury for felonies punishable by life imprisonment; I’ve revised the post accordingly.