Well yeah, of course he has presence, that's the reason I think he would be perfect for Mort; they're both a big presence in a little package. Put Mark Sheppard next to any guy tall enough to play Harry and he would look miniscule, but it would also be totally believable for him to stand up to someone who towered over him like that, and be believable that he has the power levels he has in terms of ectomancy. Sheppard also looks the part now that his hair is going, especially when he puts a little weight on. Obviously the actor playing Harry would need enough presence to compete, but that would be true next to almost any actor I would want to see cast in a Dresden series, because most of the characters have a lot of presence on the page. I mean, if I had my way James Marsters would be Thomas, and that man can overshadow plenty of actors in terms of presence. So could some of my minor casting choices, like Patrick Stewart. So I guess my point is, so long as an actor has the stature and presence to play Harry, another actor having some presence would not be a problem.
And believe me, if you just saw Mark Sheppard walking down a street, he would not look 'impressive.' I ran into him at a convention, and it took me several seconds to even recognize him. He is a small and rather average-looking man, he's just a good actor.