A term used to describe the size of a man's penis without having to give specific details. Being "in the Frankenstein zone" means you are at well above average. (8-10 inches)
The name comes from the Frankenstein monster who is very large compared to a person, and proportionally would have a very large member.
See also the Happy Zone and the Outrageous Zone.
I must say, I didn't think Greg would have a very large penis seeing as he is asian, but I was shocked to find him in the Frankenstein Zone.