The label lies differently at every brand. A tape measure does not.
Lay a t-shirt you already like flat, measure it armpit to armpit and shoulder to hem, and match those two numbers to the size chart. If you are between sizes, take the larger one — unisex runs closer to a men's cut than a fitted women's tee.
A straighter body than a women's fitted tee, a slightly wider shoulder, and a longer sleeve. Someone who normally buys fitted women's shirts and picks their usual letter here will find it roomier than expected — which is fine if that is the look, and a return if it is not.

These are 100% ringspun cotton at 180 gsm for solid colours, and 52/48 cotton-poly for heathers. Ringspun cotton is pre-shrunk, so expect under half an inch of length loss if you tumble dry on high heat, and effectively none if you wash cold and hang dry. The neck is ribbed and taped shoulder to shoulder, which is the detail that stops a crew neck stretching into a scoop.
If you can get to one of their shirts, measure it. If you cannot, a Large covers more adults than any other single size, and a slightly roomy tee reads as a style choice while a tight one does not.