We have multiple Size Guides as we have a couple of suppliers.
Each garment you are looking at should have a corresponding size guide (if not - shoot me an email as I have goofed - firstname.lastname@example.org
On a desktop it will look like this
On Mobile device, it will look like the one below
All size guides are the measurement of the garment (not the body). If you measure against something that fits well now, and compare to the guide on the garment - you should be on a winner!
See below for a groovy video that shows how to measure a garment to get the perfect size.