RE: to Ola - additional fin for my wave board
Its extremely difficult to give general fin advice since both people of a given size and fins of a given size are very different. To zoom in a bit on the subject, it would be good to know how you find your current 21.5 fin with a 5.3? If that combo is OK, a similar fin of around 22.5 would probably work OK for the 5.8. But since your approaching the max size sail the board will likely take, it might be good to go a tad bigger, especially for onshore. You can also do a lot of board tuning by looking at different types of wave fins. There are the Maui Fin RON type with a very wide base which you can choose with less depth and keep a sharp tunring board but still get more planing power. You can also choose a wave fin with a thicker profile to especially help with slow upwind work or you can go with a soft flexing thin profile fin but instead go up in size to keep a good fast carving feel and tune the board for full power sailing also with the 5.8.
Lots of choices and in the end it comes down to compromising between turning perforance and upwind/early planing and such things. A "typical" onshore wave fin at 25cm does sound a tad big, but it might work fine. The best thing would be if a friend has a big wave fin you coulr try, to see how you like the style and size.