He looked down, embarrassed for some reason at his new size. Even though he had nothing to compare to, he had a feeling he was alot bigger, and probably thicker than most. He was worried a little bit though. His inexperience meant he didn't know much about it, and whether or not Selphe would be hurt. He watched as it grew longer with her stroking, the size increasing exponentially. He was now half erect, and it was already quite big. 'Will it fit?' he wondered vaugely.
(this good? My first time of actually defining size. I like to leave that to the personal interest of the partner, and readers. ;D)