Little League requires that a bat be no more than 33" long and more than 2 5/8" in diameter. All non-wood bats must have "USA Baseball" stamped directly on the bat.
Getting the right size bat will directly impact your child's skills development and enjoyment of the game. We highly recommend NOT buying a bat that is too big, with the assumption that he/she will "grow into it."
The Length rule of thumb is to take your child's age and add 20 to it (see chart). Consider going down one inch if your child is small for their age and/or less experienced. If your child is big for his/her age and/or more experienced, consider going up one inch.
For the Weight, most players use anywhere from "Drop 5" to "Drop 12." This means that the number of ounces the bat weighs is 5 to 12 less than the number of inches long. So a 28" bat that weighs 16 oz. is considered a "Drop 12." A lighter bat will allow your child to swing the bat faster, while a heavier bat will provide more power.