Welcome to the most important choice in your new suit purchase!
Selecting the right size for you will depend on how loose or tight you want the fit to be.
All of the measurements given are the actual measurements of the suit for each indicated size. So a 6' tall person with a 42" chest would find the size large suit very snug and form fitting. However a 6' tall person with a 38" chest would find the suit roomy enough for thermal under garments.
Other things to keep in mind in size selection is the smaller the suit, you need to be more sure of your self in how snug a fit you want. The smaller the size the less rubber is there to stretch so the suit can be both difficult to don, doff and be uncomfortable in use.
In other words a person with a chest that is 1" larger than the measurements given for a size medium will have a more tight feel and difficulty donning and doffing than a person with a chest 2" larger than the X large suit because there is 8 more inches of rubber to stretch over the size difference.
If you have any questions about your new suit purchase, just give us a call.