We get it, online shopping can be so difficult! Because we carry a variety of brands, there is no universal size guide - every brand and each style can differ a bit in fit. A size medium in one brand can be a bit different than a size medium in another, and a "Boyfriend Fit" tee in a size large will fit differently than a "Slim Fit" tee in a size large, even if in the same brand.
To make things a bit easier, on every product page we 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 us and we'll try to get that to you as soon as possible.
We also understand that a description like "slouchy, relaxed fit" can mean slightly different things to different people, so again, we definitely recommend going by the listed approximate measurements on any item for best fit. Our "runs small/runs oversized" fit descriptions for each product are based on our own estimates, so please still look at the measurements to find your own best fit as every body is different :)
Got a tee you love the fit of? Measure the bust and length of that tee (laying flat, measuring across) and compare with the measurements of the product you're looking at to figure out what size you'd wear.