The smaller fin will no doubt increase control. As Roger also said, moving the mast foot forward may solve your problem. I assume you have tried this since you are looking for a smaller fin.
As far as boom height goes, there is some conflict with the above posts. As Erik suggests, it depends on how you use your harness.
I sail Formula quite a bit and hanging in the harness (weight on the mast foot) with a high boom is essential to keep the board flat and under control. At speed on a reach or upwind, if you back off the harness pressure and sheet out, one can literally take off. I use the same approach on my smaller boards - boom about neck high and weight in the harness. If I move the boom too low, I occasionally catch my harness line in the harness hook while gybing. Not a good thing.
Don't forget, it you move the mast foot up only a few cm's, you should also raise your boom a little too. If you feel "scrunched up" while in the straps, the boom is too low.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.
Last edited by Ken; 30th January 2011 at 12:02 AM.