Santiago realizes as soon as his fish truly takes the bait that he has caught a very special fish. As he works to raise and tire his catch, he wonders how large the fish truly is. Finally, the fish jumps out of the water and Santiago is able to see the magnificent creature. "'He is two feet longer than the skiff,' the old man said."
After Santiago has returned to his village with his skiff and the remains of the marlin lashed to its side, he collapsed in sleep inside his shack. However, the villagers who had not been through the ordeal with him gathered to inspect and react to what they could only imagine.
"'He was eighteen feet from nose to tail,' the fisherman who was measuring him called." So, the marlin was eighteen feet long, which means the skiff was sixteen feet in length. The name "skiff" is given to a small fishing boat, so this is a reasonable length for Santiago's boat.