You have but to inquire about bygone ages that came before you, ever since God created man on earth, from one end of heaven to the other: has anything as grand as this ever happened, or has its like ever been known?
From here the Talmud learns that when Adam was created he was the full length of the world. This means that when he would lie down, his head would be in the east and his feet would be in the west.
That distance is equal to the walking distance of 500 years.
After he sinned, Hashem reduced his size to a height of 100 amot.
Others explain this to mean that, originally, when Adam would stand on the ground his head would touched the sky. Meaning that he originally measured the distance from earth to the sky.
The Talmud ultimately clarifies that both distances from earth to the sky and from one end of the world to the other, are equal.