Jacket size?

Just look at a tag on one of her jackets. Here's the plan, she's at your place, she has to use the bathroom and then you covertly look at a tag. Simple.

Hard to say. I'm a small in the united states but the clothes I buy in Taiwan have ranged from like L to XXL :/ Best bet would be to just straight up get her measurements either by having her measure herself or getting the measurements of what she already has via brand websites. Like her medium jacket, if you know where she got it from and if they still have it online, the shop website should tell you the measurement.