Average height and weight he was nothing special to look at.  Marcus does move with an unexpected grace but its when you talk to him he that you realize under that short sandy hair is a brain.  Damn smart one.

A google search brings up someone that has worked in the field of theoretical physics for years.  Having been around egg heads before they knew why Marcus was brought in.  He was not all thumbs and could move were most people in his field of his accomplishment were old and slow.

Having a Doctorate from MIT he often thinks he is the smartest person in any room.

SGC Rumor mill
His team was recently shot up on an away mission and only two members made it back.  He had to be carried back.  Most people are surprised that he is up and about this soon and figure he was either not hurt too bad or a wuss who can't take it.