There are 43,560 square feet in a acre; this is 4,840 square yards.
You can find a conversion calculator at http://www.unitconversion.org/area/acres-to-square-foots-conversion.html Originally, the size of an acre was approximately the amount of land an ox could plow in a day. Now, I don’t know about you, but that approximation means virtually nothing to me. Today, the easiest visualization might be the sideline to sideline area of 90.75 yards of a standard american football field.
Lot sizes are commonly measured in acres or a fraction thereof. A typical suburban residential lot would measure 0.15 – 0.50 acres. As you move to more rural areas 5, 10, 20, 40, and 80 acre parcels are very common.