MAINIO kids size chart
The adult clothing usually falls into standard sizing labeling and you will find an XS, S, M, L and XL, The XS being an equivalent to UK size 6-8, S - UK 8-10, M -12-14, L - 16-18 and XL - 20-22. As mentioned above, it tends to be generous so I'd advise sizing down, i.e. if you are a small 12, go for S rather than M, if you are 14/16 - choose size M.
Here is another useful chart by Danish DANEFAE: