The rules say that skill training is tied to height, so your 7'0 PG might not imrove his outside shooting and passing as fast as a shorter player, but may improve his rebounding faster.
But for example if he is a good outside shooter and ball handler, then it may be OK to use him at PG. If he is a respectable jump shooter and range, it means exactly that, and not that he is surprisingly good for someone that tall. If he has atrocious rebounding, it doesn't mean that he will get rebounds despite this because of his height, but that even with his height he isn't very good.