For windsurfing, foil fuselages come in length from 85 to 120 cm. Why manufacturers combine different lengths with different sizes and shapes of wings for preset foils is beyond me. I have to believe that they have good reasons for that. Probably, in most cases, you’ll be good if you follow their assessment.
But, if you are not in the “most cases” category, or you just want to experiment for yourself, the adjustment in fuselage length is one of the options.
My experience: shorter fuselages are more lively and maneuverable. Lift-off is not so quick, but they are faster.
Longer fuselages are more stable, and you can make them go upwind beyond what you thought was possible.
I’ve tried 95, 100, 102, 105, 110, and 115 cm fuselages in different foils. Due to the difference in wing shapes and the way wings are attached to them, they are not all comparable. But, for example, Starboard Freeride foil comes with a 95 cm fuselage. I felt very unstable with that—too many touchdowns, too touchy for small body movements. I suppose toll people (usually, height leads to an increase in weight) will always have such problems with short fuselages. The resulting forces create longitudinal instability of the board.
So, I switched first to 105 fuselages and, in the end, 115, which worked the best with freeride foil.
For me, a shorter fuselage is ok with low aspect foils like Starboard Superflyer or Supercruiser and a more upright stance. Still, I would use 105 for those foils (or standard 102 cm, which comes with a preset original). I would use 95 cm (less than that is not an option for me) only exceptionally when the wind is stronger.
I suppose the taller and heavier you are, your preferences will go in a similar direction. With less weight and height, you could find shorter fuselages more suitable.
Well, remember, this is only my experience so far. I may be wrong, of course. 😊