When I was considering buying my first pair of Assos bibs I, too, was unsure of sizing. I had always had difficulty with PI bibs because the waist size and strap size seemed incompatible for my body. I called Competitive Cyclist and talked to one of their reps and gave him my dimensions. (Which are 5'9" & 180 lbs. He said the Unos in L would work. He was right. I'd suggest a call.