hi GREGK:QUESTION--LENGTH OF HAND HEWN TIMBERS--Well greg I have never seen them but I have heard that the pioneering UEL's that landed here in Ontario in 1784, and who had access to virgin timber, hewed plate material for buildings up to 100 feet in length.That would put the trees before hewing in excess of 120 feet standing height. No doubt many of the virgin pine would easily go the distance.
The second part of your question referred to the usage of elm for hewn material--well I did examine one barn that did have elm upper plates but that was the only time that I personally ran across elm used in this sense.
I do believe that like mice if you see one there no doubt are more, so in that sense I am sure that elm was used from time to time especially if you had no other specaes on your tract of land to use. Elm would have been hard to work with especially for plates due to all the mortising that would be necessary.
I am looking forward to what you find in those old forestry books. I have old hand books that date to 1850, that list "rock elm" in their strength tests.