I have one very similar to this, get one in the next size up from your shirt size. For example, I can fit snuggly into a small size shirt (I use small size for undershirts), so I would order a medium. Put them through the wash once and they will shrink to a nice snug fit.
i assume i should use adult sized shirts for what you are saying. im 5'4" with 33 1/2" waist so i should pick X-small then?
Well yeah... adult sized shirts, and no, unless your built like a 10 year old, then I would go with at least a small. The onesie will shrink after the first wash and will fit a lot more snuggly. You probably don't want it to be skin tight, these onesies fit more like shirts with extra fabric for crotch snaps, its not like spandex or anything. If you want to stay comfortable in it, just go with your next shirt size up (adult size). Its kinda hard to tell what size is for you based on those measurements you gave, but its a good rule of thumb to get something a bit bigger for you if your unsure, that way you could just grow into it.