I bought 3 pairs of these undies to take on a 10 day trip to Costa Rica.  I am a petite girl with slim hips but a big bottom.  I tried the medium size, which fit but was a little lose around the waist.  The size small fit best on my petite 120lb frame.  I would go for a slighty smaller size if you are in between and like your panties to have a sexier fit.  They are super soft and they dry within hours, if toweled off and then hanged.  I washed mine everyday, with just whatever soap was available and water.  They can retain BO in the crotch area, so make sure to let the soap sit there for a few minutes, if you're concerned with being extra fresh.  I wore these as regular panties and as beach bottoms, when my regular bathing suit was wet.  Well worth the money, and now that I am home I still wear them, although the panty line is a little thick.