I'm about 5'3" and 133lbs. I ordered three packs of these shirts in various colors, all in small. If you're looking for a looser fit, I agree you'd want to order a size up. However, I wanted to wear these under tee-shirts during slightly cooler weather. I usually wear size small, and these shirts fit exactly the way I want them to. They're a bit snug in the sleeves (but not bad at all), and the fit of the body of the shirt is perfect. I love the scoop neck so I don't have to try to line up the collar with the other shirt I'm wearing. The length is also great, so there's more color sticking out from under my tee-shirt. The price worked out to about $6 per shirt, which is AMAZING. (I purchased three 3-packs at $18 each.) The material of the shirts is somewhat thin, so I definitely don't recommend wearing the white shirt on its own. You could get away with wearing the darker colors on their own though. And that's another thing, the color variety in these shirts is great. I'll definitely come back for more in a year when the weather cools back off!