just checked: the boards of my Bimare are 1.2m long, the wetted length or span is 0.7m and the chord is 0.25m, hence the in water area is 0.175 sqm per board (it feels ok for 2 up sailing, but a bit too much for single handing, however I have only 14sqm sail area upwind)
Rudders have a submerged span of 0.35m and an average chord of about 0.18m.