These are lovely on but come up very large so would suggest buying at least 1 size smaller if not 2!!
I'm a 12/14 and an M is plenty big enough, maybe even too big bearing in mind they might stretch (although I've had more problems with bikini tops than bottoms in this regard) NB. I also thought these were ruched at the sides (I like the flexibility of that style) - I obviously didn't look closely enough! - but they are still a nice shape - with good bottom coverage.
I bought these to go with the tankini top. I am a 16/18 so ordered the XL as per size guide. They were too big. However I ordered the same size in other shorts and there was an inch difference in the width. These however were not comfortable and dipped at the front and back showing off the bits I was trying to cover up. They were tight on the lags and baggy elsewhere. Wouldn't recommend sorry.