Size Small (?) Fit and Flare Forever21 Crossback Dress

Pretty purple and green fit and flare dress in great pre-owned condition by Forever21. Sleeveless with a crossback, and a hole at the base of the back. Never worn, not sure of the size. Please refer to the measurements. There's thread belt loops, but no belt. Measures 17" armpit to armpit, 34" shoulder to hem. 96% cotton, 4%spandex. Machine wash cold, tumble dry low. Do not dry clean. Wash inside out.