Find an old shirt and some pants you don't wear anymore and direct them. See were the stitches line up and remove the stitching and then use the pieces as your guide, then you don't have to measure for the type of fit that you want. And if you don't have any old stuff you're willing to part with, go to a thrift store and you can usually get a deal on something as long as you don't care what it looks like. Then you can mark each piece and keep them as your guides for future projects as well. just line up the top seam of the pants to the top and stitch them together into a single piece so you don't need to start with two separate pieces when you do you're onsie. adding trims, or even cutting to add new zipper or button points is easier once you have the initial design all in one piece. That's when you can start to get fancy with it and remember, it's easier to cut away excess material than it is to add it on later from the base piece. So make the legs and sleeves a bit longer and keep trying on the piece as you go along to make sure it fits the way you want. Do some hospital style stitches to keep the onsie in one piece as safety pins can shift around and alter the form of the piece if you're not careful. Do these practice stitches along where the seams would be, but leave excess so you can make it bigger if you need to, don't cut to much off or try to be exact with your material or you may come up short in the end. Excess flaps and overly long sleeves can be cut to shape later, just be sure you have enough before you pull out the scissors. This is how I like to do it as I'm not overly fond of measuring out each piece or trying to calculate me general size preferences. but there may be online templates and youtube videos you can use if my way doesn't work for you, I'll try to link some below just in case =) .
Youtube vids, included bodysuit video for the snap closure just in case and a video on adding zippers.
baby onesie tutorial (just recalculate the dimensions to accommodate for your adult sizing needs)
bodysuit snap closure
zipper closure tutorial
Hope this helped =)