An acre is a measure of square (not linear) footage, square feet are the only units with which one can meaningfully describe an acre. In other words, an acre can be of any shape—a rectangle, a triangle, a circle, or even a star—so long as its area is exactly 43,560 square feet.

To find the linear measurements of a rectangular acre, just divide 43,560 by the number of feet you want on one side. A square-shaped acre would then be about 208.7 by 208.7 feet (because 208.7 x 208.7 = ~43,560). An acre 100 feet wide would be 435.6 feet long (100 x 435.6 = 43,560) and an acre 1 foot wide would be 43,560 feet long.

The acre, by the way, was originally an English unit of measurement that described the area that a yoke of oxen could plow in a day. It originally differed in size from one area to the next, but was ultimately fixed at 4,840 square yards, or 160 square chains (its current size).