After the Elves took many casualties in the Dagor Bragollach, Fingolfin, High King of the Noldor, rode up to the gates of Angband and challenged Melkor to single battle in such a way that he couldn't decline. Melkor being a Vala, Fingolfin clearly couldn't win, but he dealt Melkor seven wounds, survived three blows, and then when he was finally knocked down, dealt a blow to Melkor's ankle that haunted him forever.
I was searching for pictures of the battle, and I noticed that there was a huge disparity in Melkor's depicted height, from about 12 to 100+ feet, and that got me wondering how large he actually was. I looked around but couldn't find even any guesses. Being a Vala, he could have taken any shape or size he desired for the fight. With the speed-power tradeoff between the two, I've always imagined him to be about 25 feet.
Is there any indication as to how large he was for that fight, or his usual height and form?
Melkor about 12 feet tall
By EthalenSkye on DeviantArt
Melkor about 50 feet tall
By Davix92 on DeviantArt
Melkor about 100 feet tall
By juliedillon on DeviantArt