Apache Reference: mod_proxy, CacheSize
Maximum Disk Space Used for the Proxy Document Cache
Syntax: CacheSize A
Example: CacheSize 1024
Since: Apache 1.1
This directive sets the maximum disk space used by the document cache in Kilobytes. Although usage may exceed this setting, the garbage collection will subsequently delete files until the usage is at or lower than this size. Depending on the expected proxy traffic volume and CacheGcInterval, use a value that is at least 20\% to 40\% lower than the available disk space.