I was just wondering if the Mehrunes Dagon you see during the Light the Dragonfires quest is the real him or just an avatar of him (like in Morrowind).

It's the real deal. Those aren't "avatars" of the various types of Daedra coming out of those gates, but the real things. And it's the real Mehrunes Dagon tearing up the Temple District, stomping on guards and generally being a bit of a travel hazard.

Thank you for answering and in the quest did he have a definite size or could he change his size to whatever he wanted?

He has a definite size. He's a big red, four armed bringer of death and destruction. He has no need to change his size to fit through a door. He'll just rip the roof off and step over what's left of the wall.

Alright so do the other daedric princes like Sheogorath and Jyggalag also have a definite size and appearance?

Apparantly. I've always wanted to see how big Peryite would look.

As far as I know, yes. Although certain things that occur in Shivering Isles lead me to believe that they can change their appearance if they so choose.