Thanks for the advice, everyone!
I measured a couple of my favorite sweaters, one a bit more form-fitting and one a bit looser, and they were both around 20" across the chest. Double that is about 40". Since the medium for the sweater vest is 47", I figure that should give me that roomy fit without looking too big, so I ordered both colors in medium, and I will exchange for a size up or down after trying them on if necessary.
I also got these black bootcut jeans and plan to wear them with both vests, and they will well work with most tops I already own. I really like the light stitching to pick up the white in the vests. I got them in my current size and one size down because I just need to lose a few inches around the waist for the smaller size to fit (I have narrow hips, so my hips are already there, ha). It'll be a nice treat as I continue to slim down having one pair of jeans that already fits!
I'm considering these three things, plus some wall shelves for my office, my birthday present to myself this year.