unless you go measuring yourself against other guys (than taking a girls word of comparison), there's no real way to know that you have a "big" dick, especially if you're straight then you'd never go looking at another dick. besides, everyone has their own idea of what "big" is for themselves so my "big" is probably your "small"... or vice-versa.
for the longest time i never knew i had a bigger cock because of what i said above. plenty of women said i was very nice for an "asian guy" but i never really cared or took it at face value, and i still don't to this day. my wife begs to differ with me on it but i still don't care.