In Dungeons & Dragons, Demogorgon is typically described as about 18 feet tall (around 5.5 meters).

In Stranger Things, the Demogorgon from the Upside Down is portrayed much smaller: sources that analyze the show and production materials put an adult Demogorgon in roughly the 6–10 foot range (about 1.8–3 meters), depending on the scene and depiction. So the answer depends on which version you mean: about 18 feet tall in classic D&D lore, versus roughly human-to-giant size (6–10 feet) in Stranger Things.