I tried switching from a 7" to a 4.5" fin today. It was incredible the difference it made! A lot looser on the wave, worse tracking paddling in, and a LOT less stable. I was amazed how much a few inches of fin effects stability when water is running over it.
The Pro Teck 7" Super Flex fin is my current favorite, but putting on the 4.5" ups the difficulty if you're looking for a challenge.