Undersize your overcurrent protective device (OCPD), and normal operations might become impossible. Oversize your OCPD, and the whole place might burn down.
It helps that, for a given circuit, conductor sizing and OCPD sizing begin with the same two steps. Once you do them for the OCPD, you can reuse the results for the conductors.
It doesn’t matter which one you size first, but from a workflow standpoint, it’s usually better to size the OCPD first. The examples in A