I get it, online shopping can be so difficult! Because Black Salt carries a variety of brands, there is no universal size guide - every brand and each style can differ a bit in fit.
To make things a bit easier, on every product page I try to list the fit estimate (ie: runs small, runs oversized, etc) as well as measurements of every size so customers can get a better idea of what size they should purchase. If a product doesn't have a size measurement, email Black Salt and I'll try to get that to you as soon as possible.
A measuring tape is a wonderful thing for finding your best fit when online shopping!
I always recommend going by the listed approximate measurements on any item for best fit. Got a tee or pants you love the fit of? Measure the bust, waist, hips, and length of that item (laying flat, measuring across) and compare with the measurements of the product you're looking at to figure out what size would be best for you.
A lot of customers ask what size is equivalent to a certain waist size in pants. The thing is, numerical sizing (2, 4, 6, 8, 14, etc) is a made up number by the fashion industry, which is why you can be a size 4 in one brand and a size 6 in another brand. I highly recommend you measure your waist and hips with a measuring tape to find your best size. A size 25 in Rolla's will best fit someone with a true waist measurement of 25 inches around, for example.