Need help but in a onesie

Buying my first onesie but am a bit stuck on which size I will need ive seen a couple size charts but I'm not too sure how to measure properly as a couple of the charts were completely different from one another

Basically a onesie is a "T" shirt be it a commonly short sleeve or uncommonly long sleeve.
A less common design is with a turtle neck and zipper or button panel closing at the neck.
The bottom has a sort of build in pantie brief panel with snaps across the crotch piece.
With no snaps or fastening it becomes a leotard and needs a wider opening at the neck.
Since the material is stretchy like a "T" shirt this is the all important sizing component.
Sizing is not that difficult - find the upper torso of the onesie in the size of one of your
own "T" shirts - size will not be numerical but lettered S M L XL.
Onesies are generally sized for the top (chest/bust) fitting with the bottom half proportioned to fit.
If too short in the length it will make it difficult to fasten the crotch poppers same would be true
if wearing too many bulky diapers.
With too much length the problem is to your advantage - with way too much excessive length it
is possible to shorten it by sewing an internal waist seam.
One useful measurement is taking a tape measure and run it over either your right or left shoulder
and down thru the crotch and back up to mate with the end at the shoulder that will give you the
torso measurement you will need for a comfortable fit. The chest/bust is the other factor.
As for the waist the garment should be sized to unfasten the snaps and fold or move the bottom
half away for diaper changes.

If buying a onesie off ebay or etsy get the measurements from the seller or maker.
Avoid getting a too tight fitting (leotard type) for comfort although it may not be much of deal
at the chest the bottom crotch piece with the snaps Velcro or other fastening will not function
with ease and maybe come unfastened - one thing make sure the crotch piece with the snaps
is not a thong piece - normally consider a width of 10 inches for 4-5 snaps and spacing to
cover the diapers within.

View catalog views of leotards unitards and of course onesies to get a better educated fitting !

Here is one that is in auction on Ebay Search the item number:

Adult Onesie Discontinued Style Size Large Pink (331782798390)