Ancalagon is written to be as large as a mountain. That makes him about twice the size of Godzilla. But... as each of your mothers has said to me: Size isn't everything. Godzilla has fought several foes as large as himself at once and won. Now, the fight is in 1960's Japan. Ancalagon will have a flight ceiling of 15,000 ft. They may take the fight up to one mile off coast, but no further. There will be Japanese military and scientists fighting them, so ray guns, rockets, tanks, and 60's era jets.