well being adjustable its hard to give it an exact size, but really they could at least give a range
i think the mediums or large would both be fine for you, large is going to be a little more baggy (more bounces perhaps?) alone i still think they will fit fine and if you wear anything at all under them beyond that stylish thong they will fit just fine for sure, when i got my slider pants i took out the adjustments a bit.
top notch quality though, exactly what i have come to expect from dye products, not sure exactly which model i have (never really cared that much) but they were a very good investment
if your buying new just save your recipt just incase
just to give you a reference the swim trunks i use are a 31 waste and that is a comfortable minimum, i can usually squeeze my fat ass into a 30 but it is not comfortable, i go 32 on jeans with or without a belt and 34 with a belt for sure so i dont show off too much of my stylish thong...