Steph Bridge had Tarifa Fin Company blade type race fins for the North 5' 10" Rocketfish that she raced in her first PKRA races. The rear one was about 20 cm and the front ones about 16 cm fitted in the standard mini-tuttle boxes.
As you can see they are expensive.http://www.tarifafincompany.com/product ... ?Item=Race
Kitesurf Fin CNC&PHPSESSID=ft0qu4179n6dapchlequrm98t3
I doubt that they would increase the low end much, but would go upwind better.