C8 head size, but with higher capacity potential!  This light takes a 20700/21700 instead of the regular C8/C8+'s 18650.

20mm MCPCB | 20mm Driver 

What's Included:

• Convoy M21A body / lens / reflector 
• Plastic reflector insulator/spacers (1x 3535 "XP" & 1x 5050 "XM/XHP50")
• Spring
• Lanyard

Specifications:

• Body Diameter: 28mm
• Head Diameter: 44.5mm
• Length:  152mm
• Weight: 160g
• MCPCB required: 20mm
• Driver diameter required: 20mm
• Reflector opening: 9mm
• Reflector finish: Smooth 
• Battery type:  20700 / 21700