That's a grapeshot. Just under two inches.
As Tommy and others have said. http://en.wikipedia.org/wiki/Canister_shot#mediaviewer/File:MHS_canister_shot.jpg
There was also in the Civil War a Naval Canister. I'm not sure if it was bigger because of its' use so as to rip apart and splinter the wood frame and bulkhead of a ship or because a ships' cannons were always of such a large caliber the canister shot was larger. The Naval Canister was more so for destruction of a ship then ment for personal casualties .
Acey