I recently helped one of my awesome customers with sizing these skins. We based the skin off the indicated length of the ski which was 189 so we went with the Long 183 - 189cm for a nice tight fit but they found these to be a tad too short for their skies.
In asking them for some additional details on the skies they informed me of the following:
"Using these on a pair of K2 Hippy Stinx. They clearly say 189 on the side, but standing them up next to my old Atomic 191's they are exactly the same length. Perhaps the twin tips are measured a bit differently, or there is some difference between the manufacturers.
I took out a measuring tape. They are 193 cm from tip to the end of the tail measured along the curve of the ski. That explains why the skins didn't fit."
Word of advice would be to double check the actual length of the ski to best determine the size of skin needed.