What is the length of a side of a square acre?

An 'acre' is an amount of coverage of a surface. It can be any shape, as long as it covers 1 acre of surface.

If it's a square plot of land, then each side of the square is [ 208.7 feet ] long (rounded).

If it's a circular plot of land, then the diameter of the circle is [ 117.75 feet ] (rounded).

It could be a rectangle measuring [ 180 ft by 242 ft ].

Or it could be a rectangle measuring [ 360 ft by 121 ft ].

Or a rectangle measuring [ 45 ft by 968 ft ].

Or a rectangle measuring [ 87,120 ft by 6 inches ].

In each case, the area inside the boundary is 43,560 square feet. THAT'S what makes it one acre,
not any particular shape.