The most common length is in dispute. If you read Roy's article, he lists the quantities made in each length. The problem develops over the definition of RM. He doesn't count the KC guns as Registered, because they weren't eligible for the registration program. Same identical gun, with the first 250 at least having a registration number stamped in the frame cut. To the collecting community, the number stamped is good enough.
So if you exclude the KC guns, and the other guns sold to PDs all over the country as non-registered models, then the longer lengths like 6 or 6 1/2 are probably the most common. If you believe your 4" KC guns, or the others sold to PDs are RMs, then the 4" length is the most frequently encountered.
A worse issue, as you know, are the guns cut down to more useful configurations by later owners (often sent back to the factory for the work.)