A board's volume in litres — the most meaningful number when buying, because it drives buoyancy and therefore how many waves you catch.
Length and width say less than most people assume; volume says almost everything. As a rough guide for surf beginners: roughly 100% of your body weight in litres — about 75 litres at 75 kg. Intermediates ride 40–60%, pros on shortboards under 35%. The most common and most expensive beginner mistake is buying a board that's too small because it looks cooler: you barely catch waves, wreck yourself paddling, and learn more slowly for it. More volume means more waves, more waves mean more repetitions — and repetitions are what make you better. Add a little extra if your fitness is low or you surf in thick cold-water wetsuits.