JSON_STORAGE_SIZE(json_val)

This function returns the number of bytes used to store the binary
representation of a JSON document. When the argument is a JSON column,
this is the space used to store the JSON document. json_val must be a
valid JSON document or a string which can be parsed as one. In the case
where it is string, the function returns the amount of storage space in
the JSON binary representation that is created by parsing the string as
JSON and converting it to binary. It returns NULL if the argument is
NULL.

An error results when json_val is not NULL, and is not---or cannot be
successfully parsed as---a JSON document.

URL: https://dev.mysql.com/doc/refman/5.7/en/json-utility-functions.html

