How do I find output size of a network?
Yes, and it would be $124 \times 124 \times40$.

To calculate the output dimensions: 

$(dim_x - filterdim_x + 1) \times (dim_y - filterdim_y + 1) \times filters$

in your case:

$(128 - 5 + 1) \times (128 - 5 + 1) \times 40 = 124 \times 124 \times40$