I am pretty sure it says that smaug was the last dragon, but there might have been some cold drakes still around. as far as the relative strength of middle earth dragons, smaug lived in a mountain, a large mountain but not the largest. anglecon the black when he was defeated and fell from the sky he fell upon thangorodrim the largest mtns of there time, made my the debris morgoth had pulled out of his underground fortress. well when anglicon landed on the mtns he ruined them. so that sounds like to me he was a much larger dragon.