My site modx_session table size is 14GB, but only with 200 rows.
In Modx Backend Report > System Info > Database Tables > only show 2 records.
Is that normal? How should I clean this table?
Welcome to the forum 
You can truncate the modx_session table from a database management tool like PhpMyAdmin or Sequel Pro. 14GB for 200 rows seems odd though, that would be 70MB per session which is much bigger than I’d expect an average session to be (maybe 1-2kb).