So this flannel shacket turned out to be bigger than expected. It's definitely already oversized, so I would advise NOT to size up; ONLY IF you want an extra oversized look. I am just under 5'4, between 133 - 137 (my weight fluctuates a lot), I carry more weight in stomach/arms but I'm still petite-ish, and bra size: 34C (not full cup)-- if this helps. I purchased a medium but would have been just fine with a small.
This item is totally cute though, and wearable. So I definitely recommend purchasing, just don't size up!