This may help with sizing. I just bought a Blacktec shorty (from Waterskigirl). Worked great in 65 deg air/ 65 deg water the other morning. It is very different from my traditional neoprene shirts and 2/1 shorty. The material is extremely flexible and not as permeable to water. I was a bit skeptical on why the need for a 1mm shorty, but it seemed far warmer than I expected...and it is nice not to have the water "rush in".
I am 5'10" 185 lbs. Kind of thick thru the arms, shoulders, and (42") chest....so went with the XL. Seems like a good choice, as I would guess that the L would have been too tight through chest.
The inside of the shorty has a green, fuzzy, felt-like layer to it...very comfy, and dries out quickly in the sun. Attached is a close up.
(Thanks goes out to @waterskigirl)