I no longer do garment construction but I used to custom fit my own clothes and did quite a bit of costuming 40 years ago.
You can't always just make things bigger or smaller all over. There is a very real difference between someone who is a basketball player XL and when I was long torso/short armed 300 pound me or now under 200 pound XL me. That shoulder seam maybe shouldn't change as much as the arm pit, darts need to be adjusted in placement, angle and length. The neck opening maybe should be comparatively larger on the XL than on the M.
But nice thing is this isn't terribly fitted and needs to be about ease and comfort as well as protection. If you are going out and have thrift stores available to you, you might look for scrubs or other garments of about the right shape and size and use those as guides. Likewise, Walmart or comparable stores might have things in stock (scrubs again for example) where you can compare sizes and maybe luck into something on sale for $5 or under to use as a pattern.