What do you call the process of finding the magnitude of fy

The process of finding the magnitude of Fy is called vector resolution.

Explanation:

Given a vector with a magnitude equals to F which forms an angle equals to θ with the x-axis, then it can be decomposed into two smaller vectors (Fx and Fy) as follows:

Fx = F*cosθ  (placed on the x-axis)

Fy = F*sinθ  (placed on the y-axis)

This process is called vector resolution.