You are not representing the guideline for InCo very well here. Because when you sense "the biggest man" you are not sensing just a single fact about one person.
Your spell kind of has 2 steps to it:
Step 1: You are asking the spell to figure out the size of all the men inside the boundary (I assume its a boundary spell)
and
Step 2: tell you which one of the men is the biggest.
You could absolutely get a spell to determine the size of a man, even a man that you cannot see as long as you can target him in some other way, such as by a boundary target. and following that logic you also can determine the sizes of a group of men, such as all the men in a boundary.
Usually hermetic magic is pretty bad at figuring stuff out, such as "which one of the men is biggest". You could have the spell generate a list and then analyze the list yourself.
the guideline according to your citation is: "sense a specific fact about a body" and I trust your citation but i very much dont agree that sensing a specific fact about a body multiple times must allow the spell to compare the facts too.
But of course the magic mirror in your example seems to have a mind that would allow such analysis. But then again if the mirror has a mind, is a thing of magic, a faerie spirit or whatever else, then it does not have to conform to the guidelines for how hermetic magic works.
I suggest you check out this other thread about how Magic things can deviate from hermetic rules in interesting ways: thoughts on a "Magic Thing" - #13 by lvgreen