Balrog Beasts?

Unfortunately I had never read any Tolkein befor "The Fellowship.." came into theatres. So I never really had my owm interpretations of what everyone would look like. However since that movie came out i've read The Lord of The Rings, The Hobbit, and The Silmarillion all twice.

But let's talk about balrogs......What did they really look like? What makes us think that these fire maias were so huge? In the book "The Fellowship.." Legolas sees the balrog as being in the shape of a man, yet greater. And I think I might recall a passage in the Sil about balrogs being of a man shape also. The quesiton is.....were balrogs big beastly creature like beings. Or were they rather in the shape of a man, yet greater? And for the record,,,,I really don't care whether they had wings or not, however, all of us that have any brains clearly know that they did.