For deciphering what will happen to the area of cross section in such questions you can use a simple trick that is
A1L1=A2L2 . In this case let A1=A and L1=L (i.e. assume the area and length of the wire to be these before doubling its length), now you know that L2=2L1=2L an you have to find out A2 , simple just substitute these values into the equation,
so you will have AL=A2(2L) => A2 = AL/2L =A/2.
thus you can see that the area of cross section after stretching the wire to twice its length will become half of its original value . This method always works , you just have to make a small assumption for A1 and L1.
Hope that helps.