The open-back long-sleeve cover-up mini combines a crew neckline and fitted front with a fully open back. Its listing suggests beach and pool occasions, but does not specify opacity, wet use or how the back opening is secured. Use the chart to compare physical dimensions first; treat coverage as a separate photo question.

Compare a dress you can actually wear

Lay a similar long-sleeve mini flat. Record bust, shoulder width, sleeve from its stated starting point and body length. The listing gives S as 32.28/12.99/24.41/30.31 in on those four axes, M 33.86/13.39/24.8/31.1, L 36.22/13.98/25.39/31.89, and XL 37.8/14.17/25.59/32.28. Ask whether these are finished garment dimensions or suggested body values. Do not compare a flat half-width directly with a circumference.

Map the missing back measurement

On your reference garment, mark how far an open back could descend while still meeting your planned coverage. Ask the seller for a rear photo with a ruler from the neckline to the lowest open point and for any tie or closure detail. The chart provides no back depth, so a correct bust row cannot answer this question. Also request an image of the chosen color against a contrasting underlayer if opacity matters.

Set the mini hem in the planned outfit

Mark the listed body length on your reference garment from the same origin once the seller identifies it. Compare that mark while standing and sitting, using the shoes or swim layer intended for the outfit. The source calls the fabric a cotton-spandex blend but omits percentages and a stretch test; avoid assuming that a smaller row will expand to a preferred coverage level.