One source on the internet indicates there are 3 frame sizes. A J frame size, a Colt D frame size, and an XL frame size. not sure of a comparison for the XL though. Chuck Hawks indicates there are only 2 but that may be due to the age of his article. Hawks indicates
"the length and width of the medium frame are the same as those of the small frame, but the height of the frame was increased by 0.170” and the barrel boss was increased by 0.095”. The recess for the cylinder was increased by 0.055” in length and 0.140” in height to accommodate the larger cylinder of the .44 Bulldog. These are very small differences."
So I'm not sure it really matters for such small dimension increases. Though I'd still like to confirm that the Professional is built on the Bulldog frame.
"the length and width of the medium frame are the same as those of the small frame, but the height of the frame was increased by 0.170” and the barrel boss was increased by 0.095”. The recess for the cylinder was increased by 0.055” in length and 0.140” in height to accommodate the larger cylinder of the .44 Bulldog. These are very small differences."
So I'm not sure it really matters for such small dimension increases. Though I'd still like to confirm that the Professional is built on the Bulldog frame.