M Square is a great go to brand for tunics. They are made in the USA and are fine knitted "tee" fabric. This one is called their "Mix Tunic", and as you can see, it is off white and has a tie dye piece inserted both front and back with exposed stitching. Princess seaming in back.
M Square knit tunics are generally one size fits all and will. This one, to give you an idea, has a 44" bust and is 24" down the front and 28" on the longer wing sides. While roomy, this one just runs a bit smaller than their normal osfa because of the style.
Certainly will accommodate a S-XL.