If you are unsure of what size to get, please refer to this chart. I recommend measuring yourself and choosing a size that corelates the closest to your measurements. Please note that the dual sizes are a rough guide as every shop uses different measurements for their sizing.
Fleece garments allow for extra room, particurly the trousers as they are designed with extra crutch ease. For example a size 18 can fit snuggly into a size L in my fleece garments, allowing a size 20 to buy an XL.
To help, I am a large 16 in most of my clothes, on my trousers made from cotton , gingham or poly cotton I wear an XL so I can fit without being too snug. Whereas the fleece trousers I fit with plenty of room into a size L.