Each side of the field is about 35.43 feet long. 
In order to find this, we can use the formula for area of a square. 
A = s^2
In which A is equal to area and s is equal to the side length. Now we can plug in the area and solve. 
A = s^2
1255 = s^2 ----> Take the square root of both sides
35.43 = s