Bottom line..............boards don't go upwind without input from the sailor and or rig. No rig, no sailor, big fin = the board WILL NOT go upwind even if it is being blown along with a few knots of speed.
When overpowered, boards want to go upwind, but not because of the fin. When the pull on the back hand (CoE move rearwards), the sailor typically sheets out, taking weight off the mast base and puts more weight on the rear foot. This sends the board upwind, plus the sailor usually wants to lose some speed so he/she typically adds weight to the windward rail, also turning the board upwind.
When I experience a big pull on the back hand on my smaller sails & boards, if I choose, I can keep the power on, stay sheeted in, muscle the back hand through the gust, hang in the harness to keep pressure on the mast foot, use the back foot and toes to keep the board flat, and accelerated to until panic sets in. I have adjustable harness lines on my sails from 6.6 up to 11.0, so the back hand is rarely overpowered on the bigger stuff.
In overpowering conditions, it is difficult to turn the board off the wind because of the above mentioned sailor initiated dynamics, but it can be done if you are skilled (lots of TOW) and are willing to accelerate through a beam reach so that you can head off the wind. The difficulty is not so much technical, but it is very much mental.
I have raced formula in over 30 knots with a 70cm fin. Upwind is the only point of sail that offers any sanity, but only if power is continually applied and you hang in the harness lines to keep maximum pressure on the mast foot (critical for control). if you sheet out or relax, the board will do a back flip or similar nose high crash. Smaller fins help this situation, but it's the input from the sailor and rig that determines where the board is pointed or what happens.
Bottom line for Nicolo - The fin is not too large unless you are planing at 20+ knots of board speed. Even at 20+ knots, the fin may not be too big as your skills improve and if foot strap placements are eventually moved to the outward most locations. However, a smaller fin may make it easier to learn the basics of planing, jibing, foot strap and harness line use. Tell us where you are skill wise, plus the wind speeds you sail in and what sails you are using.
As Coachg said, the tail width of the board is usually the primary factor in determining fin length. If the 42cm fin is the factory provide fin, then there should be no problem.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.