I'm 5'9/140lbs and a small is perfect. Plenty of room and just the right length. The weight is definitely best for cooler weather, but I'm pretty excited about them.
Comfortable and cute
I love these sweatpants so much! They're so comfy and I love the detail down the front of them. I will say that they are very long. I'm 5'7 and usually wear a pants size 6-8 and I got the size M. If you're shorter definitely size down.