Near chuck norris's height!
There exist no predefined measurements regarding Gordon Freeman's physique, and so I highly doubt anyone can give you the exact answer. However, a rough estimate can be given by comparing the world-model of Gordon Freeman to other models and surroundings, as well as the first-person viewpoint he has in-game. Using these algorithms, one can see Gordon Freeman is just a little taller than Alyx and Dr. Kleiner, thus one can safely assume he is about just under six feet (very approximately, 180cm or so). Still, no-one can be absolutely sure, but Gordon Freeman certainly wasn't very tall; probably just over average.
188.8.131.52 20:35, May 20, 2011 (UTC)
In the chapter Point Insertion, if you go into third person mode at the bit where the metro-cop reveals he's Barney, you can lean against the measuring wall, and see that Gordon Freeman is 176 centimeters (5 ft 9).