What were the biggest logging tractors ever used? They had to be steam tractors like the one shown in the photo below. Just look at the size of this monster.
Image courtesy Forest Grove City Library, FGLpic_00063, circa 1890 - 1900's
Not only is the size of the tractor impressive, but look at the size of the load that this monster could pull. The horses shown in the background on the left put the size the tractor and trailers in perspective. I would think that the biggest limitation of these tractors would be the need for decent roads and relatively flat terrain. Still, pretty impressive.