Now that we have a number with a unit per acre we can start to work towards getting a final answer in feet. Remember when working with a unit area to square a conversion factor that is given for a measure of length.

0.00156 mi ^2 / acre * ( 5280 ft / 1 mi ) ^ 2 = 43560 ft ^2 / acre

Now that we know the number of square feet in an acre, taking the square root of the answer above will give us the length of a single side of the square.