Every child is different and unique. Some are chubby, some are slim. That's why it's hard to get the perfect fit when it comes to reuseable swim nappies. This page covers how to get the perfect fit in reuseable swim nappies, specifically the Close Pop-in Reuseable Swim Nappies.
Measure Your Child
You will want either a string and a ruler, or dressmaker's measuring tape. Take a measurement around the waist just under the belly button. Take another measurement around the top of the leg. The reason for this is that, it's the snugness that counts. Too snug, and you won't be able to pull them on. Too loose, and they lose their functionality.
Select The Best Swim Nappy Size
Then compare their measurements to this:
Select The Best Rash Top Size
Usually, but not always, the nappy and rash top sizing matches. However, here are the approximate sizes in the rash tops: