The U.S. Constitution gives a person accused of a crime the right to be tried by a jury. However, this right does not extend to petty offenses -- defined as offenses that do not carry a sentence of more than six months.
Usually, a right to a trial by jury means a 12-person jury must arrive at a unanimous decision to convict or acquit. However, a jury can consist of as few as six persons. (Williams v. Florida, U.S. Sup. Ct, 1970.)
The size of juries tends to vary depending on the seriousness of the charge. For example, California requires 12-person juries for both felony and misdemeanor trials, except that the state and defendant may agree to less than 12-person juries in misdemeanors. Florida law provides for six-person juries in noncapital cases and 12-person juries in capital cases.
In most states, a lack of unanimity is called a "hung jury" and the defendant will go free unless the prosecutor decides to retry the case. In Oregon and Louisiana, however, 12-member juries may convict or acquit on a vote of ten to two. To learn more about a criminal defendant's rights during a criminal trial, read Nolo's article Criminal Defendants' Rights During Trial: The Bill of Rights.