JERRY: Why'd you get him a gift anyway?
ELAINE: Oh, he did some dental work for me and he didn't charge me so I thought I'd get him a Christmas present.
JERRY: Yeah, well, if you're getting him anything for his birthday, I'm a large.
The context is that Elaine had given a present to a guy and then that guy "re-gifted" it to Jerry. What does this I'm a large mean?