Behold! My Morgoth Gallery!
Morgoth 1
Morgoth 2
Morgoth 3
Morgoth 4
Morgoth 5
Because of the size of the pics, They're in link format. Anyway, here's my two cents...
Being a big fan of Ulmo, I've always considered Valar around this size:
If that's too big for you, I can put it back in link form! Anyway, I consider Valar being at least over five times the size of a man. Morgoth, in my opinion, is a mixture of many evil and powerful beings. One of them being Ares, the Greek God of war.
Ares
I consider Morgoth not far from the appearance of a man, but in an 'evil incarnate' look. The terrible feeling you get from seeing him is probably part of his 'All shall fear me and despair' aura he seems to carry about him. Fingolfin must be the king of neck strength. Clearly we have much to learn from him...