Depending on how thick the metal is and on what is inside the door (foam?) you could use a few different things. A decent jig saw with a metal blade may work nicely, just drill the corner holes first. You might also use a skill saw with either an abrasive metal cutting blade or an old fine toothed carbide tipped blade. Whatever you use PLEASE use decent eye protection b/c a metal sliver in the old eyeball can ruin your whole day.
I would be concerned that a large hole like this would likely severely compromise the structural integrity of the door. Unless this is a heavy industrial metal firedoor then its structure depends upon all the parts being attached to each other and bonded to the core material (usually foam. The factory versions with a large light already installed have a decent structural metal frame around them to try to help keep them together and they probably have more reinforcement. If your foam separates from the metal sheathing then the door is pretty much toast. If you really need to do this then I would make sure to reinforce the door around the hole.