Image formation on the mirror

How can we find out how much the observer can see of himself in the mirror if the mirror is too small?

We can start to draw the optical paths from the toe to the lower edge of the mirror. Now we can see that the reflected ray (coming from the toe) misses the eyes of the observer (picture below).

Next we check if the rays coming from the knees of the observer can reach the observer's eye through the mirror. As you can see in the picture below, this is also not possible.

Of course, we could use trial and error until we find the spot that the observer can still see in the mirror. However, there is a faster possibility.