I'm 5'3" and 120 lbs, and bought the small. This dress is lovely! The waist is right at my natural waistline, the sleeves are the perfect length, and the hem of the dress stops an inch above my knee. The sewing is decent, and the fabric is very light, smooth, and comfortable. One of my best dress purchases ever, not just on this site! BEFORE BUYING, TAKE NOTE: the colors are yellower than pictured. The difference is small, but don't expect neutral tones. Also, this dress is very sheer. You're going to have to wear shorts and a camisole/nude bra underneath to keep it work/school appropriate. Finally, this particular kind of fabric is nice but can snag rather easily. Be careful which accessories and outerwear you wear with it, and steer away from things with rough plastic or zippers.