Along with the images of the apparel, each item listing will also have an image for the sizing chart. Here's an image that may help understand the measurements on the sizing chart:
As you can see in the diagram, the "Body Length" measurement is the distance in inches from the shoulder seam all the way down to the bottom of the garment, the length of the body. The "Body Width" measurement is the distance in inches from the bottom of each sleeve hole, the chest width.
The best way to check the sizing of an item is to measure a well-fitting shirt or sweatshirt you currently own- lay it flat and measure from side to side just underneath where the sleeve connects to the body of the garment and then compare to the WIDTH on the size chart. This should confirm that the item you ordered will be a good fit.
Due to the nature of production of t-shirts & apparel, each item does have a tolerance. This is shown as 1/2" in all directions from these measurements. This is because t-shirts & apparel are usually hand-sewn and there really is no exact science to how these are put together to keep them exact.
We hope that helps.