How do I limit the size of my Postgres log?
The PostgreSQL Error reporting and logging documentation states: 


  log_rotation_size (integer)  
  
  When logging_collector is enabled, this parameter determines the maximum size of an individual log file. After this many kilobytes have been emitted into a log file, a new log file will be created. Set to zero to disable size-based creation of new log files. This parameter can only be set in the postgresql.conf file or on the server command line.


So, I guess you should set this parameter to the desired value. My postgresql.conf option has the default values:

#log_rotation_size = 10MB       # Automatic rotation of logfiles will  
                # happen after that much log output.  
                # 0 disables.