It was a movie from the 90's. The guy turn into a grey and red robot, it was similar to the power rangers except there was only one person. He could grow in size while he was the robot to fight things that were larger than him. As I think I remember he looked just a ranger but he was grey and had red in the middle of the suit.

From the sounds of it and not having looked it up. It sounds like Ultra Man ?

I think the movie your asking about is called Inframan