That's not an array. That's a mapping.
Mappings can't be iterated over and they are unordered - therefore they also don't have a length. In fact, there is no way to know how many entries a mapping has, unless you have stored that information somewhere else (or calculate it from outside the blockchain by looking at transactions).
To delete an entry from a ...
I believe I misunderstood your question;
this is probably a better response:
What does the "gas" readout in Remix refer to?
The gas line is referred to the total maximum amount of gas provided with the transaction (you are including 80... gas to be used)
the execution is how much gas is effectively used of the amount provided; so in your ...