Up to you 24-29cm.

I've had Exocet Cross 90l/59cm from 2006 and Cross 94l/61cm from 2016, double rear footstraps, flat water blasting and B&J. I'd say very similar sort of boards to your JP board.
For the Cross 90 I used 5.6m & 6.5m Naish Redline 3 cam freerace, 6m Ezzy Cheetah, 5.2m Ezzy Tiger and 4.5m Ezzy Panther, plus assorted older Ezzy wave sails. On the Cross 94 I used Ezzy Lion 6.5m 2 cam instead of the Naish sails.
I've used loads of fins over the years. If I was going to buy now, I would get a Select Edge 29cm, Black Project G10 Kracken or Freewave 24cm & 26cm (if they still make them), or K4 FSW 24cm & 26cm for cheaper plastic fin. Or if you just want 1 smaller fin, a 25cm Select FX Freecarve.
Or any which look like these. Maybe not the upright Select 25cm slalom fin, it was only good on flat water & higher winds.

Techtronics Falcon powerbox fin around 27cm for speed and control.

Awesome fin when it gets rough.

5.0 to 5.8m is about right for that board. 5.4 should work best.

Swept back fin with single rear strap. More up right using double rear straps. 25-32 sounds about right depending on wind & sail size&type &local conditions. Probable a great B&J board with single strap and will prob have a good speed on double strap setup on flatter speedier conditions.