The problem with getting to know them first is you're likely to meet many guys who could be relationship-material but won't be endowed enough for your preference. The statistics just aren't in the favor of it and you probably don't feel like you've got enough time to date that many men.
If it was a girl I was meeting and she didn't want me to think her promiscuous, she would need to state it in a way that discourages me escalating on the subject. Something like "By the way, please don't get the wrong idea, but I want to be honest with you and let you know I am only interested in dating someone who's very well endowed. It's not the most important thing to me and doesn't come first before I get to know someone, but I think its only fair I let you know up front."
Make sure its a statement. Don't ask him if he's hung. If he's nervous about it, challenges your statement, or asks you how big, then you can probably assume he's not or he's too insecure for it to be any good anyways.