The wood saw on 125mm Wenger Swiss Army knives is about 100mm (4") long. It is extremely sharp, and long enough for serious firewood gathering or branch trimming.
You can use the wood saw to cut wider pieces of firewood by making two cuts in a "v" shape, working all around the outside until it is narrow enough to snap.
WARNING: Use light pressure when sawing, and be careful that the saw doesn't bind, since it can bend at the base and eventually snap off, if pushed with too much force.