Choosing the Right Fin Size
-If you are a larger, heavier surfer then you will want a fin set with more surface area to provide more hold.
-Strong, powerful surfers should consider a fin with more area
-Use a fin with more sweep if you need a smaller board to work in larger waves than the board can typically handle
-Boards with wider tails will require more fin area
-Boards with a lot of rocker
-To give a loose board more drive. A fin with more sweep or stiffer will also help.
-Use to loosen a stiff board
-To make a board more forgiving, try a fin with less sweep or more flex.
-Board with flatter rockers can get away with fins with a smaller area
-Boards with thicker rails will require less fin area
-Boards with deep channels
-Boards with narrower tails
For more guidance try this fun tool: