Hi,
I've been involved in the attempt to rehab the Clary Mill, and I'm the person who nominated it to the National Register. When preparing the nomination, I looked into the connection to the WW&F, but my visit to the museum was off season.
I'd be very happy to hear of any info you have regarding the spur at the mill, or the old trestle over Pleasant Pond Brook, which eventually became the outlet from the renamed Clary Lake.
In answer to your question, the current mill (others existed on the site dating to the 18th cent.) is 30' x 80' with the unusual characteristic for a building of that period of having the main floor a clear-span of that full dimension (no internal intermediary columns). The upper floor and attic are hung from the ridge by iron rods, and the lower story has massive columns, all resting on a checkered granite foundation.
Let me know if you'd like a visit or walk through of the mill, or need any specific documentary material. Of course, I'd love to get copies of any material any member here might have regarding the mill itself, and/or the connection to the railway.