I live in these tops (always wear them underneath unless I'm at home and it's hot). I feel as though I am a little in-between a medium and a large, maybe closer to a large. I wear a size 8 top usually (M or L depending on the type and brand). So I would buy a M if I wanted a snugger fit and L for a tad more relaxed fit. There is alot of stretch to this top. I love the fact there are lots of colors, even some multicolor which are fun. I bought about 6 of these when they were on sale because they are such a staple.
I really like this product. The fabric is substantial, and the straps don't fall down. I also like that there's no shelf bra. It's very comfortable, and it doesn't ride up throughout the day. A great base layer.