WOOD BATS MUST BE USED. BATS MUST MEET THE SPECIFICATIONS LISTED IN RULE 1.10a-c OF THE “OFFICIAL RULES OF MAJOR LEAGUE BASEBALL.”. (a) The bat shall be a smooth, round stick not more than 2.61 inches in diameter at the thickest part and not more than 42 inches in length. The bat shall be one piece of solid wood.