It all depends on the fabric really. Some are true to size and some aren't. I usually wear fitted clothes so I'm a size Small, sometimes an XS for tops. But if it's like a cotton button up shirt that doesn't give then I might need to take a Medium. For instance I bought a button up blouse from F21 and it looked like it would fit me better if I went a size up so I got a medium but it turned out that it was loose and didn't fit nicely. Also, the red top I bought is a small but it fits like a medium. The key to finding your size is to look at what the material is made out of. If there's lycra or spandex (you know the strechy material) you might need to go one size down if you like it fitted. If you like it a bit loose a small will fit nicely.