Is there an official height for the movie Balrog? I couldn't find anything, but only did a quick scan. Apparently they are supposed to be varying sizes (like Star Wars Rancors). The Wikipedia entry puts them at 12' tall, but there are other references that show them as being much taller. And in the movie scene it certainly looked larger than 12 feet tall, which is why I suggested 12' - 15' feet tall (to the top of the head, not the top of the wings). A 12-foot tall scale Balrog would be roughly 24" standing upright, which could be cheated a bit by being hunched over and the wings could be adjustable. A 15-foot tall Balrog would be 30" tall. So it would come down to what looks good, what folks would be willing to accept, and price. But in a lot of respects while it would be a massive piece, it wouldn't be need to have a lot of articulation. Sort of like SS's Jabba the Hut. Another option would be some kind of a Balrog base perhaps with the head, partial torso, partial wing and an arm surrounded by flames, like it's sinking into a fiery abyss. That way maybe you're talking about a 15" - 20" tall sculpt with no articulation and maybe a light-up feature.
I'd actually prefer a Treebeard base to a full figure.