The largest possible rectangular enclosure that the rancher can build will be a square.
To find the length of each side of the square, divide the total amount of fencing by 4 (since there are 4 sides to a square):
1000 / 4 = 250
Each side of the square will be 250 yards long, so the total area of the enclosure will be:
250 * 250 = 62,500 square yards.
Therefore, the total area of the largest possible enclosure he can build is 62,500 square yards.