As for Morgoth, I think that the same 'taller than man, but not gigantic' rule would apply to him as well.
Since he started off as one of the Valar and was the head of the whole evil operation I have always thought that he would appear physically bigger and stronger than Sauron or the Balrogs. However, this can be taken too far. I've seen paintings of Morgoth where he is the size of a skyscraper. Yet again this brings up the problem of how did he move around inside Angband or get out the front door to fight Fingolfin. On the other hand he apparently did try to stomp on Fingolfin, and that would explain why Fingolfin was unable to cut off Morgoth's foot, if Morgoth's foot was bigger than Fingolfin.
But seriously, probably about a 15' Morgoth is a pretty realistic guess. Which if you follow my line of reasoning would put the cap on Sauron and the Balrogs at about 15', if not less (I tend to think less).
If you were the Dark Lord you would not want your minions to be more physically impressive than you, right? That's sort of what happened in the Ungoliant situation. She grew much more powerful than Morgoth, had pretty much beaten him, until he was rescued by his balrogs.
Perhaps Ungoliant is the creature that I imagine to have eventually reached being about 50' tall, so that she would be able to so easily defeat Morgoth. Why then was she so easily cowed by the Balrogs? Well, for one thing all of them were together at once. Assuming that there were seven, which I know that not everyone does, that is seven nasty 'demons of might' with nasssty flaming whips slashing at her beautiful webs and then at her. I kind of imagine Ungoliant to be rather cowardly at the best of times, and I doubt that she was immune to the power of terror that the Balrogs could put into the hearts of their enemies.
So Ungoliant, seeing that she is being assailed by at least seven 'demons of might' and one now freed and very ticked 'Dark Enemy of the World,' even though she was 50' tall, I hardly blame her for turning tail and bolting. She was outnumbered (even if they were much smaller than she was) and her enemies were armed with fire which had rather easily destroyed her webs and could probably do the same to her.
Hmm, this got rather longer than I'd intended so I'll stop now.
