The standard length for firewood is 16 inches. The reason for this is that firewood is typically sold by the cord or fractions of a cord, an amount of wood that measures 4 feet high by 4 feet wide by 8 feet long. Even though most fireplaces can fit a piece of wood longer than 16 inches, firewood is cut to fit in the cord and not the fireplace. A tree is usually cut up into 8-foot logs, a length easy to haul out of the forest. These are then cut up into six 16-inch "rounds," which are split into firewood.
Hold a tape measure at one end of the fallen tree and stretch it out 8 feet. Cut the tree three-quarters of the way through with the chainsaw at this mark, and then roll the tree so the bottom becomes the top. Cut down to the previous cut until the log falls from the tree. Repeat this process until the rest of tree has been cut up into 8-foot logs.
Hold the tape measure at the end of one of the 8-foot logs and mark the log every 16 inches with the axe blade. Cut the log at each mark by first cutting on one side and then rolling the log over to cut through it on the other side. Repeat this process for all of the 8-foot logs.
Flip the 16-inch log rounds upright so that you can see the cross-cut of the wood grain. Grip the axe handle and chop up the log round into pieces of wood that are no thicker than 8 inches. Repeat this process until you've chopped up all of the log rounds into firewood.