Hi Jerome, I've put an 8.1 and 7.5 on mine and must say that they "work". The KA117 is with 6.1/6.8 (and wind that fully powers those sails) in a really fun freeride/wave modus. I've used a 5.0 as well and that still works, although by then the board does get a bit big. With larger sails its more like riding a carve and just "cruising" catching the occasional wavefront to "pop". As far as I know the starsurfer 117 is identical to the A117 (predecessor of KA117) (don't know about a starsurfer 140). If she isn't too heavy it should be fun. Just one thing: if you bought the wood version I would try and talk your "learning girlfriend" into learning a bit more on a different board until you are sure she won't fall on it or have the mast whack the tip of/off the board... Wood looks nice but it isn't something made to last long in/on a surfboard, repairs take long and are exspensive.
Fins: different people different tastes.
I weigh 80 and use the stock freeride fin (380) with 6.1 and 6.8 and I like it (no tailwalking, good enough speed (usually don't wear GPS on KA but when then 27 ~ 30 kts which is much faster than most people are prepared to (can) go in those conditions, haven't seen any steam behind me though
) through the waves (< 1.5 m)). Still haven't heard that anybody was happy with the wave fin.
Have fun