Our shirts are measured laying flat without pulling or stretching.  The width is from under one arm to under the other arm.  The length is measured on the back of the shirt, from the base of the collar to the tail.  Some slight variations in sizing do occur from shirt to shirt, so if you are in any doubt, then go with the larger size.  Please keep in mind that the measurements on the chart below are not chest measurements.  The surest way of getting the best fit is to use a shirt you currently have that fits the way you like and then lay it flat, measure it, and compare it to the chart below.  Also remember that the shirts are straight, so they will measure the same width all the way to the bottom, which is the classic tropical fit and shape.

Measurements