The kite's area in m². Chosen by wind strength and body weight — too big is more dangerous than too small.
As a rough guide, a 75 kg rider needs about 9–12 m² in 15–20 knots, closer to 7–9 m² in 20–25 knots, and 12–14 m² below 15 knots. Heavier riders go up a size. Two things matter more than the table: first, an overpowered kite is the leading cause of serious injury — when in doubt take the smaller size, since you can still ride an underpowered kite but you can't land an oversized one. Second, conditions count: gusty offshore wind demands far more margin than steady sea breeze. Most riders cover 80% of their days with two sizes.