I disagree. His height, in of itself, would not keep him from being a good center in the NFL. There may be other factors however that concern the coaches which have precluded moving him there. Some taller linemen have good flexibility and are able to get low and maintain leverage. If St.Clair does not have the flex to offset his height, it would prevent him from playing with good leverage where ever he lined up. When he played center at UVa, I thought he looked good in this regard, or as I recall anyway.
Remember, Jeff Dellenbach did a good job for us as a center for several seasons, and he is also 6'5" plus. IMO, Dellenbach was a much better center than Ruddy was.