Size and strength advantage obviously goes to Optimus, but Iron Man has the advantage in speed and agility. I don't see regular repulsor blasts doing anything other than annoying Optimus, but he is going to have a hell of a time simply tagging Iron Man.
I think Tony can win this if he fights cleverly and uses all his gear well, but if Optimus actually gets his hands on Tony, he will have to change his name to Iron Can. Because he will get squashed like one.