--experimental_disk_cache_gc_max_size=<size in bytes, optionally followed by a K, M, G or T multiplier>
If set to a positive value, the disk cache will be periodically garbage collected to stay under this size. If set in conjunction with --experimental_disk_cache_gc_max_age, both criteria are applied. Garbage collection occurrs in the background once the server has become idle, as determined by the --experimental_disk_cache_gc_idle_delay flag.