Where I grew up a lot of guys had big dicks so I never got that much attention for it until college. Still my favorite goes like this:
I was sleeping in my dorm room and my roommate was gone for the weekend, I got a knock at the door and it was a girl I had made out with a few times before. I could tell she had a few drinks in her so I wasn't going to take advantage of that so I just sat on my bed while she sat in a chair and we talked. She blurted out at one point "Do you have a big cock?" to which I gave a surprised "What?!" and she responded with "Well, you're a big guy so I was just wondering if you have a big cock?" I went the easy route and said I didn't know. She told me she would be right back and when she got back to my room had a ruler and a tape measure in her hand with a big grin on her face. So I under the covers measured him because by this point I was getting sort of aroused. So I told her the results, just a hair under 8 long and a little over six around. She scoffed and said I was lying and just as she did i pulled off the blanket. It was like magnets were in her hands. She just reached out for it with both hands and started stroking it. She asked after about 20 seconds if she could touch it...which was obviously a little belated but I naturally said yes. She muttered to herself the whole time things like "so fucking huge" "it's as big as my wrist" "it's making me so wet" "it feels so nice" eventually she asked if she could suck it and the second she put it in her mouth she let out a moan of relief. She fingered herself while sucking me and came quickly. I don't usually cum from blowjobs so i pulled out and then jacked off in her mouth which she absolutely loved. She had a propensity to get drunk a lot and ended up telling everyone on my floor (including the resident advisors) about my cock size, often times right in front of me while grabbing at my package. Really an awesome time.

Others amongst my favorites are:
upon feeling my bulge "What's in your pants!?" and upon seeing it "Oh my god! (covers mouth and then once over the shock sucks it down)

in a hushed tone "Gaawd.." to which I said "what?" and she replied "huh? I didn't say anything" "yes you did, you said god" "Oh (sheepishly)...i didn't mean to say that outloud..it's just...really big"

(feeling me press against her as I'm about to try and go inside of her with no foreplay, looks down) "wow...that's....impressive"

"yes! enough of that small dick shit!" ( I know her ex boyfriend and he is a total ass so this one was personally satisfying on a different level)

"you ARE big aren't you"

"It's the size of the boat! It's the size of the boat!" being screamed while I was giving it to her nice and hard. I love dating girls with a sense of the humor.