What I've discovered is that if you try your best to act rather normal about it, people won't stare for too long. They'll assume something normal (whoa, that woman isn't exactly lucky - I first thought it was a man! ... or pssh, poor guy, his wife is even making him buy her panties ... something like that) before long.
Also, about sizes, I'd say that women's sizes are slightly, but not very, smaller than men's. My Small male boxers are on the big size even though they still fit, and the Small girl's underwear fits quite well.
And seriously, don't worry about people seeing it. Just bring your pants down so they're inside them, simply. Not to mention that people don't usually look under the stalls.
