An acre is equal to the following:

4046.86 square meters
4840 square yards (exactly)
43,560 square feet (exactly)

So, if you consider the area as a square and find the square root of each item above, you'll find that one acre is equal to a square of:

63.62 meters x 63.62 meters
69.57 yards x 69.57 yards
208.71 feet x 208.71 feet

But, the most standard shape for an acre is one furlong by one chain or 660 feet by 66 feet (660 x 66 = 43,560 square feet).