Measurements are in inches. "Body Width" is chest circumference measurement. See photos below for more descriptions.
Here are photos that we took to give some fit examples. The size chart is accurate for the "Body Width" in the fact that it gives the actual shirt measurements right under the armpit hole, and then for it to fit, it stretches a bit around you. For example, if I match up my measurements (shown in photos) with the size chart I would think that maybe an XL would be what I need, but as you can see below, the XL starts to get a bit big for me in the chest and straps areas. Also as an example of the opposite, the small is very fitted on me, but this shirt is stretchy enough to where it is still comfy. The size chart is not accurate for the "Body Length" measurements. This shirt is long but not as long as it says, saying the measurement is from the back neckline to the bottom hem. It would be more of the measurement from about the nape of your neck, not the actual seam of the shirt. (click images to enlarge if needed)