The biggest I have ever deepthroated in its entirety is 10.5 inches. It belonged to a divorced man in his mid-sixties who I used to orally service on a repeat basis. He was also quite thick too at 6.75 or so. I know that the only reason why I was able to take all of him, as hung and as thick as he is, is because of the pliability of his cock. Although he got erect, his cock was more spongy than it was rock hard, making it easier to stuff and accomodate all of him in my mouth.
It did take a few tries before I could begin to deepthroat him. I'd say it was a good several years of sucking him on a repeat basis before I was able to handle all of him. Even then, I gargled some Lidocaine to numb out my mouth and throat. When I swallowed him to the hilt for the first time, his eyes literally rolled back. Apparently, before me, only two others have been able to deepthroat him. He was so appreciative, and I was only happy to be of service to him.