Here is the size chart for selecting the Watery Reptile Core wetsuit for men. The measurements are taken with the suit laid flat, without stretching. The wetsuit will naturally stretch and adjust to your body both with use and in water, providing a snug fit during swimming.
If you find yourself in between two sizes, choose the one that matches most of your measurements. However, the most important measurements are generally chest and height (ankle to shoulder).
A wetsuit should not be too loose; if there are many air pockets or "bunching" around the suit, it is too large for you.