How to pick the right brush head size for you
Pick the correct brush head size for you based on the user's age, or you can base it on the actual size of the user's mouth as well. To measure the size of your mouth, you can use dental floss to determine the right size for you.
Kid's Sizes

Adult's Sizes:

Sizing and Measurements
3-4 (prev 3-7) | 47.8 mm wide; 28.0 mm long |
5-7 | 45.4 mm wide; 38.3 mm long |
8-10 | 52.4 mm wide; 42.5 mm long |
11-13 (prev 8+) | 59.9 mm wide; 59.9 mm long |
Adult's Standard | 62.1 mm wide; 59.9 mm long |
Adult's XL | 77.4 mm wide; 59 mm long |