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

20mm MCPCB | 22mm Driver 

What's Included:

• Convoy M21E body / green-coated lens / reflector 
• Plastic reflector insulator/spacers (1x 3535 "XP" & 1x 5050 "XM/XHP50")
• Screws
• Lanyard

Specifications:

• Body Diameter: 27.3mm
• Head Diameter: 45.5mm
• Length:  146.5mm
• Weight: 140g
• MCPCB required: 20mm
• Driver diameter required: 22mm
• Reflector opening: 9mm
• Reflector finish: Smooth 
• Battery type:  21700