Angie, I wasn't sure about sizing because this was my first experience with Everlane. So I ordered the S (instead of XS) and was glad I did. The reviews were helpful -- people about my height and weight had mentioned which size they purchased and how it fit. Interestingly, it is no longer on me than it is on the model, who is a lot taller and is wearing the same size! Go figure.
The fabric is not substantial but very light. For some wearers, the dress is too sheer. For my purposes, it is not -- I won't be shining a light through it and mainly intend to wear as a kind of "work at home on hot days" in my non-AC'd house, though I will also wear it out for casual walks and for travel. Also, I don't mind if people see the outline of my legs -- it still feels more covered and modest than short shorts.
I think this dress would be too lightweight for the Pacific Northwest. It is ideal for a hot, humid climate. It would also make a great bathing suit cover up because it would dry quickly.