I can see this being useful in, a LOT of campaigns since the majority of monsters and NPCs are size Medium or Larger.  It feels like something most useful to a Rogue, Wizard, or Warlock.  How were you imagining this feat being utilized?

I imagined that rogues would definitely pick this up. There was a scene in the first season of Critical Role where a gnome member of the party pretends to be the son of another member of the party in a fact finding mission. I wanted to give players who enjoyed that form of deception a mechanical reward for it. I also imagine that only certain creatures, who are knowledgeable or wise, might not overlook a halfling or a gnome, not thinking of it as a serious threat.

Yes, I can see that.  Incidentally, I think it could work with children of Medium-sized creatures as well.  Not that many people would want to RP as the child of another PC, but I could see a similar scenario occurring.