I ordered a large, which states it fits chest sizes 40-42, although some sites have this exact make/model stating it fits 40 to 44 inch chest sizes. I have a 40 inch chest, I'm 6'3", and approximately 180 lbs. The large fits perfectly, and looks trim and not bulky. It is exactly what I wanted and I'm amazed I didn't have to order a size up, as many of the reviews state. Based on this purchase, the sizing is exactly what the company stated. My personal view: I watched some videos on life jacket sizing, and you need to be able to fasten all zippers and buckles while feeling snug, but not too tight. Life jackets should hug the body snug with a little grip, because otherwise the water will lift if off the body if it's too loose, making it flop all over the place. You're supposed to sit on the floor, legs extended outward, and let someone grab the shoulder sleeves with their hands and lift upward as if pulling it off over your head. If the shoulder sleeves rise upward over the ears, it's (supposedly) too big. It will lift up over the head when completely submerged in water, so you need a different size, or at least to make it much tighter (or try a different brand/make/model if you can't find the right fit. I bought two of these life jackets, both large, and they both fit identically and perfectly. I returned the color I didn't really want because the one I wanted came back in stock just after ordering my second choice. Sizing was consistent for both, fitting like a glove with a great style and comfort. Based on the other reviews (and I read them all over the course of three weeks before pulling the trigger on this purchase), I get the impression that a large portion of the population thinks it's smaller in body size than it actually is. I don't think most people actually know their true chest size or their waste size, unless they've been to a tailor in recent history; and clothing companies have adjusted sizes each year to accommodate the "body positivity" agenda on humanity. Clothes marked "large" five years ago are not labeled "medium." If you put on a couple of pounds, it's going to that area for most men (man boobs and upper arm fat) in tandem with love handles. Both spots are hugely affected when it comes to finding a proper life jacket. Anyway, for those medium-tall or taller guys who are also lean with healthy bodies and low BMI (meaning healthy, not just "skinny" or skin and bones), a large size should fit very well for someone in the height range of 5'10" tp 6'3" and in the weight range of 170 to 190 lbs, with a chest size range of 40 to 42 inches, and a waste between 32 and 34 inches. I think a 44 inch chest size would be bordering on too snug, there's a little wiggle room once you break-it-in and the neoprene relaxes after a few water adventures. I freaking love this life jacket.