Basically 12 gauge(a little less) or more correctly 12 bore. It was the size of the smooth bore muskets of the day. Using a round ball that size is not as impressive as it sounds. But It it still is quite impressive.
With the same bore smooth bore you like the Brown Bess one could launch 4 rounds per minute. The Baker? 2 rounds per minute. It is a specialty weapon used for sniping. We did the same thing with the Kentucky (or more correctly Pennsylvania ) Rifles.
A better British rifle of the era was the Ferguson breech loaded rifle.
Although the musket had a higher rate of fire, how many accurate shots could a rifle shoot before the other side got within effective musket range? And how many musket shots would actually get fired before the other side either charged to bayonet range, or ran out of musket range?