Here's my two cents worth.. The right fin depends on so many different things.

Bottom shape.. tail shape.. tail width.. the tail rocker and also how you like to surf eg down the line fast or hard tight turns.

Even the wave size and shape can also make a difference as to what fin work best.

I would look at 6" as a min and 9" as a max.. maybe go around 7-8" to start with.

Some like the slidy feel of small fins but I don't like it.. I like to turn off the rail and have a fin that will stop any slide or slip.

With my windsurfing boards I'd always go as small as I can because I don't like the feel of really big fins but going too small is also a bad feeling..

It's hard to explain.. You might end up trying a few different fins before you find one that feels just right for you.

DJ

Thanks for the feedback DJ and Rodriguez , Yeah that tail measurement is 17.5 inches, flatish rocker in the middle ,4" of tail kick and a huge tail v and double concaves . this makes me sound like I know what I'm doing .Anyhow ive got a 10 '" fin out of one of those superseded boards ,you know ,the ones you lie down to paddle on and a new 8" one from shapers coming soon ,so hopefully I might be able to experiment a bit .LSD post a pic of rice bubbles fins ,I figure you've done the most of the experimental work anyway . cheers Rupert