I don't know my pie slice skeg size translates to fin type size, but for reference my skegs average 24 square inches.

In planning your skeg I would error on the large side. Being adjustable you can just drop it and forget it in most situations. If a condition arises where it is too small your fun quotient will diminish. If it is too big you do not have to drop it full down.

Here is my experience with a seventeen foot, hard chine kayak in howling wind with ninety percent white caps on three to four foot wind waves:

With the skeg up the boat turns hard into the wind. Forward progress is little to none as course correction is a full time job. In other words this can be
life threatening.
With the skeg full down there is a slight amount of lee cocking. This is fairly easy to correct with edging, but maintaining a course adds nothing to the fun
of playing in interesting conditions.
Adjusting the skeg to somewhere between half down and three quarters down holds the course perfectly.

~Bob