Historically, an acre is 66 feet by 660 feet. However, an acre is a measurement of area, so the length and width can vary as long as the total area is 43,560 square feet.
660 feet is one-eighth of a mile, also known as a furlong, which is the length oxen can pull a plow without resting. A land is 16.5 feet wide or 10 down and back trips along the furlong with a standard 10 inch wide plow. A farmer could reasonably plow two lands in the morning, and two lands in the afternoon, which totals the acre measurement. An acre was historically the area of land a farmer could work in a day.