Why do girls lie about it?

Girls always say size doesn't matter bullshit. I'm not buying it. My gf said size is huge in a relationship. if a guy is under 6in then he's pretty much gonna die alone. Im pretty sure my gf might be a ***** hahaha isn't average like 5 and a half or whatever sounds like a lot of guys are screwed. Hahaha what do you girks think about the subject. I hear girls like 8X6in