Smooth skin neoprene brief.

[TysonZA]

there is a 2 way stretch yes,

Size wize, I originally decided to half downsize as per the ladies advice, I normally wear a 38" but I went to a 37" which was perfect, and they were more than happy to resize it for me.
But since I bought an extra pair off Ebay which was a 36"and that fitted to just a little tight.

So my advice is half downsize if you want a normal fit, downsize if you are going to use it for waterpolo or just prefer a tighter fit.