The big front wing of the foil — it makes the lift. For beginners, around 1,500 cm² on a surf foil and 1,100 to 1,700 cm² on a windfoil: more area forgives more.
The front wing decides whether your first sessions are fun: more area means lift at lower speed, so you fly earlier and get away with more timing errors. The classic mistake is buying the small, sporty wing straight away — it's faster, but it demands speed you simply don't have yet. Start big; going smaller comes on its own as your skill grows. Mistake two is looking only at the square centimetres: how a foil feels is often decided by aspect ratio, the relationship between span and chord. Low aspect means short, thick and stable; high aspect means efficient and fast, but considerably more demanding. What often gets missed: the wing never works alone. Mast length and fuselage change the behaviour just as much — so only swap wings once the one you have is genuinely holding you back.