Do girls lie about dick size when they're with a guy? Would a girl ever say "Anon your dick is nice and big" if it was really just average or so?
learn about non verbal communication
if you think you have a large dick and you make an innocent sounding joke about your dick being small and she looks at you like pic related then you have a large dick.
Why does it matter? If you really want to know if you're bigger or smaller than average, there are statistics to be found on the internet. I can't speak for all girls, but personally I really don't think it would matter, as long as it's not a micro-penis or some sort of elephant monster cock. And ever then it probably wouldn't be a deal-breaker if I really liked the guy.