I am usually a size small up top (5'5" 130 lbs), but this shirt runs quite large. Had to order an XS, but even that is a little too roomy. That said, no one wants a tight or restrictive field shirt, so an XS works well for me. Breathable, quick drying, easy to clean (the shirt in the beige color looked rough after two days of bushwhacking through an African jungle- totally covered in dirt, some blood (got scraped up), and vegetation stains!- but one quick hand wash in a sink with some soap and all the stains came out).