Haha, I'm seriously considering using Christian Bale as a reference for Morgoth now!
Thanks for the suggestions everyone! I tend to agree that Morgoth was not ridiculously tall like Ted Nasmith painted him, but, if I'm not mistaken, the words "tower" and "towering" are used to describe the way he appears. He should appear as tall as makes physical sense (keeping in mind the things that
skip pointed out), perhaps even larger for the purposes of art, because
Legate is right: if Morgoth looks like Fingolfin's equal the drama is lost.
I plan to be tricky and arrange my drawing in such a way that the heights of the two combatants cannot be compared, though I still have to make Morgoth appear towering and menacing. I'm not sure if I can pull that off, but I'll try. I think the face will be the most challenging aspect to draw right.