Troxel Legacy Lavender Antiquus Slim Profile All Purpose Helmet 7 Mesh Covered Vents. Shell - ABS (Upper/Lower) GPS II Dial Fit System. Slim Profile Silhouette Fit Small Up to 21 5/8" Hat Size Up to 6 7/8 Medium 22 - 22 3/4 Hat Size 7  7 1/4 Large 22 3/4 + Hat Size 7 1/4 +