I'd say it runs true-to-size. I'm a size 2 - 4 and am 5'3." I ordered the Small and it will be roomy enough to put a bulky sweater under it when the weather gets colder. The sleeves ran a little long on me, but I don't mind that because I can always roll them up.
This jacket is worth all 5 stars, especially since I got it 20% off by searching for an Outdoor Research promo code! I agree with all the other reviewers that it is lightweight and breathable (by raincoat standards). I've even used the armpit vents when riding my bike around Chicago and it was quite a blessing.
My only complaint is the collar. The collar is the place that the hood folds into when not in use. I don't have problem with the design, I have a problem with the material. Outdoor Research used a very soft fleece-like material for the lining which is quite warm. Because it makes my neck hot, I'm not sure I'll be wearing this through the summer.
I still think the jacket is worth the full five star review because I don't think any jacket would be cool enough for Chicago summer rainstorms -- I'll just grab my umbrella instead.