bro, this is simpler than you think. dominant sexual men do this all the time without coming off a pervert, but rather sexually confident.
Scenario: You meet a woman in a bar and you getting to know her in the [FIRST 5 minutes] you CAN and SHOULD direct the conversation towards sex. And mentioning your dick size would be easy but before you go there, there is a few things you must do and communicate:
Those things are
1. be a fun friendly guy who laughs and teases her
2. be confident
3. when she make smart comments or ask questions don't justify yourself. e.g. (if she asks what did you do today as a way of finding out if you have a job, you say something like: I just came back from narnia. I'll take you there sometime *laugh*)
Ok here is how you do it... (See what I write here and then tell a REAL story that happened to you)
Woman: "I just came back from Bermuda" ....blah... blah
You: [follow off the conversation where she leaves off]
Hahahaah! You went to the beach, I take it? How was that?
Woman: why are you laughing?
You: tell me about your beach story first then I will tell you mine.
Woman: I met some ppl...had margaritas...blah...blaj
You: Well, I never went to Bermuda but this one time I went was upstate NY at a lake on a man-made beach. I was having sex with the ex-girl friend. And that particular night she kept complaining it was hurting her, come to think of it she always complained.[right here you are hinting you have a big dick] Anyways we ....blah...blah...and then I raise up my head and saw these sweaty white rednecks hiding in the bushes looking at us. We got the fuck out of there fast as possible.
Key to accomplishing your goal:
* your story should be funny