When I import the .stl file into the slicer program it is too small / big?

Scale it up or down by a factor of 10. I’ve not looked into how to fix this as it doesn’t bother me all that much.

Where are the side fins?

I only run symmetrical foils and usually reglass my fin boxes in straight. However, don’t worry about reglassing your fin boxes in, these fins will work just fine with the usual toe in setup. If you need a reason why then read my book.

The fins are ridged from 3D printing. What do I do?

Wet and dry sand paper. Sand them with water. Start with 240 grit and work your way out to 600 grit. 600 grit is about the same as micro ridging used on high performance planes and boats.
If you want gloss then keep going. 1500 grit and 2000 grit. Also known as glass paper.
It should come up matt in finish.
Then use cutting compound to make it shiny. If you’re really O.C.D. use toothpaste as a super fine cutting compound and then car polish to get that mirror finish.
Use the palms of your hands rather than your finger tips. Finger tips rub grooves into the surface.

How do I choose the correct fin length?

You can’t. The fin length is not really as critical as the fin area. This is also known as the fin loading. We look at the loading per square cm (also written as cm2 , cm^2, sq,cm). Thus you body weight divided by the area will give a fin loading.

For example if someone weighs 70kg and the fin is 86cm2 they’d have

3 fins * 86 cm2 = 258cm2

70 / 258 = 0.2713

That translates to a fin loading of 0.27 kg/cm2
This is useful if trying to compare your fin to another person whom is a different weight to yourself. Although in a nutshell, surfing is a very solo sport and all you really need to know is the fin area when choosing your fins as your weight does not change.

If there are 2 people trying to test equivalent fins, one with a mass of 70kg and the other with a mass of 100kg. We can take the above 0.27kg/cm2 and multiply it out.

0.27kg/cm2 * 100kg = 270cm2
270cm2 / 3 fins = 90cm2

Thus the 100kg person will need each fin to have an area of 90cm2.
This is a rough estimate as direct scaling in foil design can not be properly done this way, but for a quick rough estimate it’s good enough.

Does fin twist and fin flex affect my fin sizing?

Yes. Both twist and flex will reduce the lift and therefore you can move to a larger fin area to get the same lift.