That's an interesting question. There's been something similar covered by people recently in
this topic. To cite my personal opinion from there, I think the size of Morgoth and Sauron and even Maiar in general (think Balrog!) was more about the impression than about physical height. I don't know how to express it so that it is perfectly clear, but hope you understand what I mean when I say it like this: it is not that you would put Sauron or Morgoth on a scale and say "21 feet, 800 pounds, next please". I am sure, for example, that the Orcs did not have to build special new entrance and raise ceilings in Cirith Ungol for the possibility that Sauron might come to an inspection and would not fit into the rooms. If he really came (of course, we are talking all hypothetically here), he would fit - somehow. But when you saw him, you could not be sure, and at that moment his presence would overwhelm you to the point that you won't be able to think of how tall he exactly is. And once you were away from his presence and tried to imagine, you'd think "well, I am sure he was taller than me - a lot - he was taller than the door - terrible..." but again, you won't have anything that will help you exactly determine the height. It is as if you had some spooky experience, like that you are walking in a forest at night and suddenly something big and black with shining eyes appears between the trees... and it was only your neighbor's dog who might not be as big, but the impression in your mind is a lot different, and if you tried to determine how exactly big it was, you cannot do it from your memory. That's the impression I get from the description of Balrog, and I think it will be very fitting to see Sauron or Morgoth the same.