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Gboots said..
Colas ....for dummies like me....are you saying just two fins ....or two large fins and two small trailer fins as a quad ?
Two large fins would be best, but since the front boxes on boards designed for thruster or quad setups are too far forward, it means that when pushing in turns you may end up pushing too far back for the fin, oversteering, and killing the speed. So you must add rear fin(s) to stabilize your turning radius. How big or how many depends on your technique and conditions, the more your add, the stabler the turn, but the bigger the drag. In general the small trailers sold with the twin + trailer sets (e.g. Mark Richards, Al Merrick, ...) are a good starting point.
The wider the tail, the more you can have big back fins, as you can keep your rear foot firmly on the kickpad. On Simmons shapes you can even have the twin fins just on the tail, and I guess it could work also with the twins in the back boxes on wide tailed boards with a quad setup.
And the lighter and shorter the board, the more efficient it is to be pumped in turns, and the more critical the fins become. Fin differences are less obvious on longer boards. In riding short boards (7') I feel I use the fins as main wings on a plane, whereas with 8'+ boards, I use the rail as the main "wing", the fins are more like the tail wings of a plane, to give directional control, so they are less critical.
Adding a central trailer fin to a wide tailed board means that the tail will release easier when on a rail than 2 quad trailers, but will stabilize the board yaw like feathers on an arrow during the rail-to rail transitions. You may like it or not. 2 small trailers will be more efficient at holding a line on the rail in wide tailed boards, but may give a "shopping trolley cart" impression between turns where the nose of the board veers right/left a bit uncontrollably.
In a nutshell, experiment as wildly as possible. In small weak waves you cannot really ruin your session, you can make anything kind of work, and it will always be interesting and educative, and can carry spare fin sets with you. Experimenting in good waves can be frustrating, though.