Le Creuset do not list pan volumes on there site so for Round Cast Iron:

Diameter Volume
18cm 1.8 Litres
20cm 2.4 Litres
22cm 3.3 Litres
24cm 4.2 Litres
26cm 5.5 Litres
28cm 6.7 Litres
30cm 8.4 Litres
34cm 12.4 Litres