What to look for when sizing a wetsuit
You need to know that the top of the line Cell Gold and Bionik have a longer torso to accommodate tall people, like a lot of the top athletes. If you are a 6’2″ person trying to get into a medium DNA chances are the torso is going to be too short and the suit too tight, especially in the back. You will put a lot of tension at the bottom of the zipper and the neoprene may give up after too much stretching. This athlete will be a lot more comfortable in a Cell Gold or Bionik medium.
Long legs are not so important because you can pull the legs of the suit a little over the calves without causing any tension on the suit or discomfort for the swimmer.
The sleeves need to come above the wristwatch and snug under the armpits. When the athlete is in the suit have them cross their arms in front and see how tight the neoprene is between the pectoral and the shoulders. If you have too many wrinkles and the neoprene is too loose, this means that the suit may be too big. Try one size under.
Make sure the neoprene is nice and snug around the arms, especially for the ladies who sometimes have thin arms.
For sleeveless models, the ladies need to be two sizes under to make sure the neoprene is nice and tight in front. Otherwise when they go in the water the neoprene will expand and cause a lot of water drag resulting in poor performance.
The provided sizing chart is only a guide for you to start with. However, the best way to know if the size is correct is to look if the neoprene is loose on the chest and arms and see how the athlete feels in the suit. If their chest is too compressed you may have to go one size up.