As of January 1, 2018, the new USA Baseball Bat Standard was implemented. USABat Standard bats must be used in the Little League Major Baseball Division and below. 

Rule 1.10 - Baseball

The bat must be a baseball bat which meets Little League specifications and standards as noted in this rule. It shall be a smooth, rounded stick and made of wood or of material and color tested and proved acceptable to Little League standards.

Little League (Majors) and below:

It shall not be more than thirty-three (33) inches in length nor more than two and one-quarter (2¼) inches in diameter. Non-wood bats shall be labeled with a BPF (bat performance factor) of 1.15 or less;

EXCEPTION: For the Little League (Majors) and below, for regular season play and Tournament, composite bats are prohibited unless approved by Little League International.

Click here for Little League's Bat Resource Page