Yes, you can specify a size limit for the object cache. Value range can be between 0 - 2047 megabytes.
The following is an example of how to configure a 500 megabyte limit :
Blue Coat SG#config t
Blue Coat SG#(config)caching
Blue Coat SG#(config caching)max-cache-size 500
Blue Coat SG#(config caching)view
Estimated access freshness is 100.0%
Let the ProxySG Appliance manage refresh bandwidth
Current bandwidth used is 0 kilobits/sec
Policies:
Do not cache objects larger than 500 megabytes
Cache negative responses for 0 minutes
Let the ProxySG Appliance manage freshness
FTP caching:
Caching FTP objects is enabled
FTP objects with last modified date, cached for 10% of last modified time
FTP objects without last modified date, initially cached for 24 hours