area under the phase encoding gradient in MRI
Actually, it is as simple as the frequency encoding gradient. Both encoding gradients look terribly different, but in the end they work the very same way.


The outer $k$-space position $k_\textrm{max}$ is related to your resolution, e.g. $1\,\textrm{mm}$, or in terms of the FOV: $FOV/N$ where $N$ is the number of image pixels.
For the steps between two $k$-space lines, $\Delta k = 1/FOV$


With $k(t) = \frac{\gamma}{2\pi}\int_0^t G(t^\prime)\,\textrm{d}t^\prime$ you can find that for the largest gradient moment $M_\textrm{max} = \frac{\pi(N-1)}{\gamma FOV}$. The steps in between scale this down linearly. 

Pay attention to the line numbering, because the central line in $k$-space should be acquired with zero gradient moment.