Decide how you plan to use your bo. Demonstration requires a different approach from the speed/strength needed for combat training. If you're a new user, ask for help from your instructor or a more experienced peer before choosing your bo.
Choose a length suitable to your personal use. Most people usually choose a staff a few inches shorter or longer than their height. Shorter staffs are more manageable, therefore more preferable for demonstration. Longer staffs give more extension to your reach, making them better for combat.
Grip the bo at its widest point (usually the middle) and fill how it fits in your clenched fist. If a strong, firm fist can be made then the width is fine.
Select your bo style. Staffs come in a variety of hard and soft woods, rubber or foam, with or without coating. The key to bo demonstration events is to look good, so lightweight bo staffs are preferred for demonstration because they allow for more speed and control. Heavy-weight staffs are the best way to develop speed and strength. Stick fighting can cause damage, so soft woods and rubber foam staffs are good options for avoiding serious injury.