I like him at guard. A lot actually if we get him mid second or so where we are likely to pick. At center, he's likely a work in progress.
The height doesn't bother me. Peter Konz, Mike Pouncey and Max Unger are 6'5. Nick Mangold, Chris Myers, Alex Mack and Maurkice Pouncey are 6'4. At the combine, Frederick will likely measure in at 6'5 or less.
What gives me concern at center is his lack of experience at the position. But at guard I think he'd be a great run blocker. Pass pro can need some work, but I'd be okay with him starting at guard and moving to center later on in his career (if at all)
Greatest Plank Ever!