Here is the size chart for selecting Watery Duncan wetsuits for children. For these wetsuits, your age, chest, waist, hips, and height are taken into account when finding your correct size.
Usually, size is chosen based on age, but the most important measurements are waist and chest size. A wetsuit should ideally not fit too loosely. If there are many air pockets or "bags" around the suit, it is too big for you.