[JimmyR]

Smith and Wesson and Colt revolvers designate their frame sizes with letters. I don't know much about Colt, but I can tell you about Smith and Wesson frames. From smallest to biggest:

I/M- itty bitty frames, not in production anymore, most chambered in 32 S&W Long or similar.
J- Examples include the Model 36, 642, etc. Smallest S&W in current production, commonly chambered in 38 special, with some chambered in 357.
K- Examples include the Models 10, 19, 66, etc. Mid size revolver, often used as a duty weapon back in the revolver hay day. Typically chambered in 38 special or 357
L- Examples include the Models 586, 686. Mostly modern revolvers chambered in 357, with variations in autopistol calibers as well.
N- Examples include the Models 28, 29. Also used as duty revolvers, and typically chambered in 41 mag, 44 mag, or 45 ACP. These have a long history, and include some of the oldest cartridge revolvers.
X- Models 460 and 500-
Z- Governor