Greg, the medium should do just fine. Below in the question answer section, it looks like the regular length Pinnacle will squeeze into the small, which means it will have extra space in the size medium stuff sack, that extra space being your long.

I can confirm that the Pinnacle Long fits nicely into the medium Event Sea to Summit compression sack. When really compressed, it forms sort of a roundish melon shape.

I can confirm that a long will fit into a medium eVent Sea to Summit compression sack. Plenty of room.