The answer to this is that Buck weighs 140 pounds. This can be found in the first chapter of the book. So this is what he weighs while he is still a pet dog back in California. He is said to be smaller than his father. His father was a St. Bernard, but his mother was a Scotch shepherd dog and that is why Buck is not as big as his father was.
At one point in Chapter 5, we see that Buck has lost a bunch of weight. At that point, he has gotten down to 115 pounds.