FG's are short all the time. In fact on long FG's they often put a guy in the end zone because you can return it if you want to, and there's lots of running room.
Here's an example:
http://www.youtube.com/watch?v=Qt8FUjk13Y8So what if Hester was 7 feet tall and could jump to block the kick. Not even for the prospect of returning it but for the defense. I'd think a guy like Randy Moss could block something that was low enough.
Watch this clip, this is what sparked my curiosity.
http://www.youtube.com/watch?v=NfD25zFCxWgedit to add: from the comments to the YouTube link:
"Goal tending by a defensive player leaping up to deflect a kick as it passes above the crossbar of a goal post is prohibited. The Referee could award three points for a palpably unfair act. As a result, you will not find anyone in the NFL attempting the strategy you appear to suggest."
Not sure how valid that is.