Can't you just simplify the model to decrease the number of triangles so that the ratio between triangles and pixels is more even?

Is there a physical limit as to how many pixels a triangle can be? Can you have a triangle smaller than 1 pixel-area?

The tradeoff between triangle size (and thus number of triangles) and fragment size is what is shown here, so simplifying the model is a form of navigating that tradeoff. My guess is you can have triangles smaller than a pixel, though whether coloring that triangle / performing is-in-triangle tests depends on your algorithm.

Shading complex character models is super neat, and there are a wide variety of cheats to do so without losing resolution in the texture map as well. Check out this GDC talk for more info: https://youtu.be/yhGjCzxJV3E