I don't follow the show, but from what little I've seen, penis size seems to come up on How I Met Your Mother with some frequency. Three things I can remember:
1. Colbie Smulders tells the Ted character that he's bigger than some other guy she dated.
2. Colbie Smulders to NPH: "Your penis is enormous..". It's actually just a part of his exaggerated retelling of a story or something. Later an older woman (someone's mom, I believe?) says the same thing, in another "didn't really happen" scenario.
3. Smulders and Alyson Hannigan talk about how some guy that Smulders is dating (?) has a small penis. They talk about it pretty matter of factly, as it's not big deal, iirc, but the guys make a huge deal out of the women discussing such things, feel bad for the guy etc..
4. This is some random short clip that showed up as a suggestion when I searched youtube for the other ones: