How long of saw do you need to cut a log? The US Forest Service has stated that the saw needs to be twice the diameter plus a little bit (6”). They do not say where they got that requirement but it is clear it was not from actual practice in the woods. I'll call this the USFS rule and it sounds good on paper but not what was done in the day when crosscuts were king. They do not indicate a different length for two man or single sawing. Based on what was used in the day and what works the USFS rule would require a saw much longer then is really needed. The rule should be that the saw length should be at least the diameter of the tree plus a minimum stroke length, maybe 18" to 24" for two man sawing. For one man sawing the saw could be as short as the diameter of the log plus a little to protect your hands from hitting the tree. This seems like it would not work because the center teeth of the saw will never clear the kerf at all. However with the proper technique it works quite well and has worked for well for over a hundred years. The technique is that the sawyer pulls the saw into the kerf on the pull stroke and just rides the saw with no force into the kerf on the push stroke. This action will make the saw cut only out from the center of the tree and keeps the saw gullets clear. The other thing that was a common practice was to put some lead in the rakers near the end of the saw so that the saw would tend to cut more on the ends and pull shaving toward the end of the saw rather then pulling shaving into the center of the tree.
In the posted photo of the long saw I figure that the tree is nearly 18' diameter at the cut and the saw is about 22' long. This would allow for a 4' stroke and the teeth in the middle 14' of cut would never clear the kerf.
Another thing to consider with the twice the diameter plus a little bit rule is that the sawyers would have to have a stoke length at least equal to the diameter of the tree. With larger trees, especially on springboards this would not be possible.