I'm female (obviously

), just turned 23, over 5'6" but under 5'7", around 135 pounds, and what size I wear REALLY depends on the designer! I shop in SO many different places and not all sizes were created equal.
I wear a 5-7 in most juniors things (occasionally a 9) and Pac Sun stuff. I wear a 4 in Abercrombie & Fitch, a 6 in American Eagle, a 4 (sometimes a 6) in Gap, a 27 in Silver, a 28 in 7 for All Mankind, a 3-5 in Wet Seal, and *GASP* a TWO in Banana Republic and NY&CO. HOWEVER, due to all the muscle mass in my legs (and trust me, I see striations, so it IS muscle), it can be tough finding pants that fit me sometimes -- on certain occasions, I have to buy bigger sizes just to get 'em over my legs. I've bought 8's, 10's, and even 11's that fit just fine in certain places.
I agree about not letting size dictate your success. I have plenty of friends who "appear" to be "fatter" than I am yet they wear smaller sizes. I've never been below a 2-4 even at my TINIEST (105-115 range for awhile -- I looked like a junkie and was very unhealthy, I also had perpetual dark circles and sunken eyes) because my hips and bone structure just won't allow it.