Short answer:
Plugin - No
Tool - For enterprise there is bundled charts and information on cache size/ + hits / misses, for open source/community there is none provided by Kong or created by 3rd party community members.
Maybe OpenResty platform provides some analysis tools?
I imagine there is probably a way to do this, I too would like a simple real time api call(plugin/admin api) that just returns size of cache and free space left (in bytes or kb). Something I will look into down the road.