Your fin has a massive effect on the board's behaviour. It has to be compatible with the sail you're using, the strength of the wind, your body weight and also the style of board. This might seem like a lot of variables to consider, so take it step by step, starting with the basic principle of matching the fin size to the size of your sail. Work through the procedure here, entering your details for each section, press the Calculate Fin Size button, and the script will calculate the ideal fin size for the conditions. Provided you are within 1-3cm of these recommendations you'll be in the right zone for your fin size.