what about L the length of the wires? what happens if I increase the length of wire A and keep the length of wire B the same as before? doesn't that mean that the force on A is now greater?
Yes, the force on the wire is found by integrating ${\bf J}\times{\bf B}$ over the volume of the wire, and increasing the length of the wire increases its volume, so the force on the wire is greater.
and if it is greater does that mean newtons 3rd law doesn't apply?
No, the force on wire B also increases, because increasing the length of wire A increases the B field in the vicinity of wire B.
Note: If wire A is already much longer than wire B, both effects will be very small. Only if wire A is smaller or similar in size to B will there be a strong increase in the forces from increasing the length of A.