I tried it on in the evening, with one of the shirts I plan on wearing with it, and wore it for a while, and the fit actually seems fine.

Thanks for the reassurance, folks. It was a bit unnerving to go from "Oboy oboy it's here it's here!" to "O drat! Did I order the wrong size?"

But now I've taken out the basting stitches and presses out a few wrinkles, and I'm good to go!