The perimeter of a polygon is the distance around it.
In the case of a square, since all its sides are equal, the perimeter is given by:

where a is the length of each side. Therefore the perimeter is:

Two quantities are directly proportional if as one amount increases, another amount increases at the same rate. With this in mind we conclude that the perimeter of a square is proportional to its side length.
If a=6 then the perimter is:

Therefore if the length is 6 the perimeter is 24.