I ordered small in these as that's the size I take in everything else. However, these are super small - far smaller and tighter than the size guide would suggest they should be.
Having bought these trunks twice before and being very happy with them I looked at the label in the ones I have and ordered the same size - Large. Despite my waist size coming down two inches in the intervening period (to 36) these new large ones are very tight. Currently deliberating whether to send them back and reorder X Large. Great product though, just be careful of the size you order