From the web‑based manager you can go to System > Config > Advanced > Disk Management to edit the WAN optimization & Web Cache storage and change the allocation size to limit the amount of storage available for WAN optimization byte caching and web caching. The size is in Mbytes.

You can use the following command to change the size of any WAN optimization storage location. For example, in the FortiGate-51B the default WAN optimization storage is Internal. Use the following command to limit the amount of space allocated for WAN optimization to 20 Gbytes