The coin is slightly larger than a 50 cent piece.
HISTORY OF THE CHALLENGE COIN
The tradition of the challenge coin surfaced during the World War I Era when the practice of carrying a coin or medallion designed for a specific unit was popular with the Army Special Forces and was later adopted by the Air Force, Navy and Marines. The soldiers were to carry the coins at all times and present it when “challenged” to prove their affiliation with their unit or branch of service. The absence of a coin to prove your affiliation when “challenged” resulted in numerous consequences. The most popular consequence required the hapless soldier to buy a round of drinks for the challenging unit. This practice continues today to be as popular as it was back in WW I.