I had a hard time deciding a size due to the differences in sizing and measurements between the US and the UK. I took a chance and ordered a 12 (for reference I am anywhere between a 6 and 10 in US sizing pants/shorts (depending on brand), M in leggings —including Waterlust —if looking for a comparison). The 12 fits well, though I haven’t swam in them yet. I love the rise of these leggings over other swim leggings I have. I don’t understand the premise of the pockets, personally, and as someone with bigger calves, they are a tiny bit snug (not uncomfortably and I do not think sizing up would help) in the lower leg.
Hope this helps someone who was looking for size comparisons like I was.