By the way, it's
navel like the orange, not
naval as in ships and sailors and maritime stuff..!
The domet flannel shirt of the Union/Federals, for instance, was apparently a sort of "one size fits most" type of affair... Big guys had the cuffs a bit past the elbow, small guys rolled up the cuffs or sleeves... Again, small wonder that many preferred a shirt or two from home... Wambaugh's shirts reflect that.
I sent Wambaugh some measurements for a custom Jacksonian-era jacket I had 'em make for me. The material was uber-expensive logwood-dyed jean wool stuff, so they called me after the passage of several weeks right before they actually cut out the material... I was chagrined that I'd actually gotten an inch or two bigger around my already rotund bierbauch/ beer belly, but they made it to size and sent it out. You pay for the tailored garments, but they do fit.
Follow the measurements guides of whoever you are ordering from. If they say to measure around your natural waist at the navel, then do it. If they mean your hips, then do that. Inseam, around the thigh, etc. etc. Almost all pants I wear for living history are a "standard" 20-inch thigh, which is "correct" for Civil War issue kersey wool pants, but my legs are much skinnier, no more than 17 inches at the thickest part of my thigh. So they are a bit baggy. I wear my Federal trowsers too big by design... What a recruit would have been given. I hemmed them myself, and I'm by no means a very good sewer, but they have my crude stitching on them.
I got a Federal sack coat unlined, and I'll be danged but the sleeves are just a bit too short when I extend my arms out, but it fits pretty good overall otherwise. For me it is often the shoulders that presents a problem. Look at the measurement guides at Wambaugh and maybe even a non-Civil War living history organization or two like Townsends in Indiana or Samson Historical, also in Indiana, and see where/how they take measurements. Be sure to understand that modern blue jeans typically conform to so-called "vanity" measurements. For instance, I war 29x30-in. blue jeans, but my "bauch" or paunch is pushing 36-inches these days, and my belly is about 34 inches, and my chest is about 38 inches. So reenactor trowsers are a "true" 34-in. waist, not blue jeans "vanity measurements!"