When shopping for a bat, it's important to know what size barrel your league permits and the weight/length allowances. The number will appear on the end of each stick for the weight/length. While Little league is flexible, high school and college ball follow strict rules.
The number generally appears as a negative number, such as (-8) or (-11), which indicates weight minus the length. So if your bat is a 30″-19 oz., it will have a lightweight drop (-11) on the barrel.