There is only one area=2*5=10 sq km. However, if you are going to change the lengths of the sides while keeping the perimeter the same then the perimeter is 2(2+5)=14. The maximum area is when the sides are equal so 4a=14 where a is the square side length, a=3.5km and the area is 12.25km. This requires a factor of 3.5/2=1.75 or a 75% increase in the shorter side. The decrease on the longer side is 3.5/5=0.70 a reduction of 30%.
The minimum area is zero as the shorter side shrinks to zero while the longer side approaches 7 km. The minimum area is when the shorter side reaches its permissible minimum limit. If this was, say, 1 metre=0.001km, the area would be 6.999*0.001=0.006999 sq km.