Set the default value for max-size parameter on any cache_dir.
The value is specified in bytes, and the default is 4 MB.
If you wish to get a high BYTES hit ratio, you should probably
increase this (one 32 MB object hit counts for 3200 10KB
hits).
If you wish to increase hit ratio more than you want to
save bandwidth you should leave this low.
NOTE: if using the LFUDA replacement policy you should increase
this value to maximize the byte hit rate improvement of LFUDA!
See cache_replacement_policy for a discussion of this policy.