The Hollywood Reporter actually dug into it, based on a bronze handprint in Madame Tussaud’s museum. The results:
The scientifically objective results: Trump does indeed have hands just below average size, particularly for a man standing 6-foot-2. According to various human anatomy websites, the average-height American adult male (5-foot-10) has an average hand size (measured from the tip of the middle finger to the wrist) of 7.44 inches. Trump's measures 7.25 inches. (His shoe size, incidentally, is reportedly 12, but THR editors have so far been unable to locate a Trump footprint).
Hillary shouldn’t use this, but others should. Late night comedy especially should, relentlessly. Nothing would bother him more and drive him nuts. Hell, make him sue the HR, and get his hands measured in court.