Personally I would not recommend using the word size or telling him that he's big. Instead, say "Your dick feels so good inside me..." He just needs to know that is satisfying you. Above all, avoid all mention of previous men you've slept with who may have had bigger dicks. If he asks you if you've been with a guy with a really big dick and are you avoiding telling him that out of courtesy or because you don't want him to feel badly, I would say something like "Different men I've been with have had dicks of different sizes, but, honestly, yours feels better than any of them because a) it is the perfect size for my body and b) you know how to use it, which not all guys do." End of discussion.