NGINX cache. Real meaning of zone_size

Hello, I read from the list

“zone_size” is a size of keys_zone, i.e. shared memory zone used to
store cache keys (some minimal metadata about cached pages, about 64
bytes on 32-bit platforms).

but what does this practically mean ?

Let say a new request arrive and the request has to be cached. What does
NGINX do if the “space” in the shared memory is over ? It delete the
oldest file in the cache (and the key too) ?

Is there a rule to size this parameter ?