This product is a specific logo for Bujinkan - an international martial arts organization based in Japan and headed by Masaaki Hatsumi, it is best known for its association with ninjutsu (according to Wikipedia!).
Dimensions (LxWxH): 19mm (diameter) x 4mm (approx.)
Weight per cufflink: approx. 5g (light)
Size = x mm (W x H)