How to get full query text from performance_schema.threads?
As I read the manual, ... (In other words, I did not test the following.)
It cannot be set dynamically; the manual is clear on that.  Instead, you should be able to set performance_schema_max_sql_text_length in the config file (my.cnf, etc).  That would require a restart of the server.
Note: The setting is limited to 1MB.