According to the United States Public Land Survey System, a section of land is approximately 1 mile by 1 mile and consists of 640 acres. There is no fixed number of acres per side because an acre does not have to be square as long as it 43,560 square feet.
A section of land is approximately one square mile - as opposed to exactly one square mile - because of land survey errors, difficult terrain and the curvature of the earth. For example, a township is 36 sections in a 6 by 6 square. In the United States and Canada, the sections in the north and west of the township are more likely to be inaccurate.