In SGOS V6.5.9.9  /  V6.6.5.1 or later software,  the maximum value you can specify for max-cache-size in the ProxySG appliance CLI and Management Console is dynamically configured based on the capability of the ProxySG model.  For example, for an SG300-25-PR unit, the max-cache-size is 55993 MB.  Previously, 32256 MB was the maximum value.
 
The default max-cache-size value is 10000 MB for newly installed software from 4096 MB.   If the case of an upgrade from an old version, the value remains the same.