Since I've been married for quite a long time, I don't necessarily go around meeting other women. However when we were swinging I did have to get to know other women at least a little bit before we had sex, but the subject of dick size or body parts never came up prior to that act. Since we also never put nude pictures on any profiles we had on personal sites (nor sent them to others even if requested), none of of the women had any idea of my anatomical features before the clothes came off. Of course since I usually had no idea what their nude body looked like either ahead of time all was a complete surprise and part of the appeal of first time play.
While others have had their opinion of my dick (including some of the women when they did see it), I really don't consider myself anything special, so I'd have no reason to discuss or "warn" them ahead of time. All of them were easily able to handle it and I'm sure many of them might have had men larger than myself, so there would be no point in it and in my opinion it would just make any man sound like a braggart; which would especially be pathetic if the woman didn't agree with his assessment.