The "best" answer for this Yahoo Answers post says to go a size up for the racing suits, so maybe use that as a guide when ordering briefs:
https://answers.yahoo.com/question/i...4143503AA42PdJ
I spent a year in Italy and when I went shopping, I found that going a size up from what you'd buy in the States is generally best, so that's what I'd personally recommend for Jaked since they're Italian.