Multi-Stage Common-Source Amplifier - When is it no longer a small-signal?
It is no longer small signal when you decide it is no longer small signal.
Strictly, &quot;small signal&quot; analysis means you're finding the linearized behavior of the system at an operating point -- i.e., \$dx_{out}/dx_{in}\$, and it's only valid for \$\lim_{\Delta x_{in} \to 0}\$.
In a more general sense -- how much distortion can you tolerate or ignore?  It's &quot;small signal&quot; up to that level of distortion.