Hi Nico,
Remember with the Blast you have a fairly narrow board, 66cm - a lot of other brands have 2-4cm more width on their Freeride boards - so when not planing, the Blast will tend to feel smaller - but the FW is not a narrow centre board, it does have a more parallel outline like the original Stubby and Blast, but is still fairly "standard" in centre width compared to the other 2 shapes. So I would say with the 95 FW, 2020 model, that you should be fine on 4.7-5.9 in 24 knots upwards. if that 24knots is sometimes more like 20 knots, then rather take the 105, I don´t think it will feel too big, no. Centre fin I would try the original setup first, if you don´t manage to make it work for you, then go for a 28-29cm size as a centre fin, similar style to the fin on our FW 115? good luck on the forwards/catapults! :-)