The size of fin you choose depends on the type of board you are riding, your style of surfing, and your weight/size.

Generally speaking, high-performance boards with 2+1 setups use a middle fin between 7.5 and 9 inches in length, with shorter fins being easier for smaller surfers to turn, while longer fins lend themselves to more powerful turns and stability while on the nose.

Traditional single-fin logs typically use fins between 9 and 11 inches long, with longer fins providing more stability while noseriding but sacrificing maneuverability, particularly for smaller surfers or those without a lot of power. 

Fin placement depends on the type of waves you are surfing and the feel you want from your board. Generally speaking, the farther forward a fin is, the “looser” (more maneuverable) it is, but the less stability it provides in critical sections, such as in the barrel or when noseriding.

Sliding the fin back adds more control but makes the board more difficult to turn, which can especially be an issue on longboards, which are already somewhat large and unwieldy. Noseriders typically have the fin as far back as it can go, while boards that are intended to be surfed aggressively and with a modern flair might see the fin moved forward in the box (toward the nose) to loosen it up.

That being said, different people like different feels, so it’sa good idea to experiment and move the fin around a bit between sessions to see what works best for you.

Generally speaking, longer fins provide more hold and control, while shorter fins are easier to turn. Meanwhile, wider fins provide more drive but tend to be more difficult to maneuver. That being said, there are a number of other factors that affect how a fin performs, including outline, rake, foil, and flex.

Flex is often built into the final 1/3 of a fin’s tip, and provides a lively, springy feel through turns. This is due to the fins ability to flex during the turn, then snap back into position as the turn is completed, projecting the board into the next section. That being said, flexible fins are not ideal for noseriding, as they sacrifice stability when on the nose.