Board Thread:Warhammer 40k General Discussion/@comment-25618639-20150627031101/@comment-25177957-20150628065402

The Emperor put on many guises throughout his lifetime with his psychic abilities, the disguse of Revelation when he met Uriah, or the disguse he put on when he found his son Leman Russ.

It's possible the Emperor was at least 10 feet tall, he could psychically alter his apperance, use his extensive knowledge of technology and science, it was his armor, or he naturally grew that way. Since Astartes are tall and they are built off of the Emperor's genetics, the Emperor's height must certainly have been genetic.