I read the other day Short stubby fins at the back for pivot, bigger rakeier fins further up in the box for carvier drawn out turns. And start with 1 inch per foot of board.
Depends what you want.
And I've seen a few places that say 8.5inches from the tail to the rear trailing edge is a good starting point for fins.
On my 7'4 single egg I use a 7.5 with a fair bit of flex, bout an inch from the back of the box. Or a 10" pushed right up in the box when it gets bigger.
And don't worry bout the shed, a dirty shed means you've spent plenty of time testing fins


Now get ready to pay the fine....

Pup?.