135 lbs. 5' 6". Athletic build. I went with the XS and it's a little loose fitting, but very well cut and I like the length in the back.
I picked up one of these shirts in a store to wear around the office and really like it. There aren't many women's short sleeve plaid shirts that look nice and I appreciate that this is not cut like men's shirts. They are lightweight and look great with jeans. I ended up picking up another one soon after.