Another image has been leaked which may give us a better indication of the height of an Imperial Knight:
This photo suggests that the Knight may be about an inch taller than a Trygon. The Trygon and the Riptide are very nearly the same height, which would make the Knight slightly taller than I had previously guessed. However, I suspect that the Knight Paladin is actually in the foreground of the image, making it look slightly larger than it actually is. I think I'll be standing by my guess of a 6.5 to 7 inch tall Imperial Knight.
I also neglected to mention the rumors that the Imperial Knight codex (whether it's in the form of a White Dwarf or possibly a Dataslate) is supposed to contain instructions for fielding an Imperial Knights army, a Mechanicum army, or a Freeblades mercenary army. Unfortunately, I've been unable to find out what any of that means. I'm not sure if it's a fluff-only difference or if it affects force organization.