COSSIE+CO sizing is very simple - swimsuits are sized 0 to 4. Below is a conversion chart with comparable sizing. 
  SIZE
  0 1 2 3 4
INT XS S M L XL
UK 6 8 10 12 14
EU 34 36 38 40 42
US 2 4 6 8 10
AU 6 8 10 12 14