The size or diameter is 3 inches. A 3" 50 caliber round is shot from a bbl 50 times the diameter of the round (50 calibers) or 150 inches/12 = 12 1/2 foot barrel. The round is 35 inches in length.

Hope this clears up any question on size. I found a Britishicon ordnance site with very helpful people who have helped out.

Bruce