Small D dimensions: 14.75" at widest point. 12.5" front to back at the center. Fits Breeze and similar sized fixed seats.
Large C dimensions:14.75" at widest point. 13.75" front to back at the center. Fits Solistice, Equinox, Prana, Sisu, and Karla kayaks, as well as some previously discontinued models like Storm, Siricco, etc.
XL Revolution Current Design adjustable Revolution seat. 
Can be attached using the plastic screws from your previous seat, or Plastic Ribbed Push Rivets, part number 6KES036.