I should add that for the last 25 years I have raced longboards, course slalom and formula on upwind and downwind courses. I have always tried to maximize my planing and upwind performance with the largest fin I could handle, so I am pretty used to and like the larger fins. However, if free sailing is the main focus with only earlier planing as the goal, I would recommend a 50 or 52 without hesitation.
Your body weight makes a difference. If you are in the 90kg range, you could go bigger.