Enter your body, your skill and the waves you actually surf. The number is litres of foam, the measure every brand now prints on its boards, from a model fitted to twenty published calculators and guides rather than any one brand's catalogue.
Sweet spot with a ±5 % working range. Rusty Preisendorfer puts shaping and cut variation at about 5 %; advanced surfers feel 1 L, pros half a litre. Pick a stock size that lands inside the range, not on the exact number.
Curious where the number comes from? See how the industry would size you, with every constant in the model and the sources behind it.