How to alter system on AWS RDS?
The RDS Postgres instance has a parameter group. Check in RDS colsole what parameter group is defined: RDS -> Databases -> choose your db -> Configuration -> parameter group. If it's the default one, create a new one and change track_activity_query_size to the value you need. Apply the new parameter group or change the actual one. The parameter is static so a restart is needed in any case. 

Check this link for more details, please: 
https://docs.aws.amazon.com/AmazonRDS/latest/UserGuide/USER_WorkingWithParamGroups.html