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