How to increase the size of query audit log?
For DBA_HIST_SQLTEXT, you need Enterprise Edition with the Diagnostic Pack. If you have that licensed, you can enable collection of historical data with:

alter system set control_management_pack_access='DIAG';

You can control the retention time of this data with:

begin
  dbms_workload_repository.modify_snapshot_settings
  (
    retention =&gt; 5184000
  );
end;
/


Where 5184000 is 60 days (5184000 minutes).