C8 head size, but with higher capacity potential!  This light takes a 21700 instead of the regular C8/C8+'s 18650.

20mm MCPCB | 22mm Driver 

What's Included:

• Convoy M21B body - Tan / AR lens / reflector 
• Plastic reflector insulator/spacers (1x 3535 "XP")
• Screws
• Lanyard

Specifications:

• Body Diameter: 27.1mm
• Head Diameter: 35.7mm
• Length:  141.5mm
• Weight: 120g
• MCPCB required: 20mm
• Driver diameter required: 22mm
• Reflector opening: 9mm
• Reflector finish: Smooth 
• Battery type:  21700