Fin length depends on many factors. A 42cm fin on a 130 liter board may not be too big unless the board is moving at 20+ knots of speed. Certainly if the water is shallow, a shorter fin may be required. There is little relationship between sail size and fin size for a beginner or novice that isn't planing or just beginning to plane. Speed is the issue. In fact, the larger fin can be an asset in light winds, helping keep the board upwind while slogging or planing assuming there is no dagger board.
I use a 42 cm fin on my iS 111 (108 liters) with sails down to 6.6 while hitting top speeds of 31 knots (GPS). Would a smaller fin be better at these speeds? Sure, but upwind issues are generally not related to fins, but instead technique.
Roger's foot/weight placement suggestions should solve the problem. Longer fins can help get you upwind, but don't necessarily cause you to head upwind if your foot/weight placement is correct.
Someone out there has a good old weed fin and my be willing to sell and ship it to Italy.
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.