Truncate table at a certain size.

Hello,

What would be the best method to truncate a table once it reaches a
certain size.   

For instance, a table named log.  I can check the size of the log;
  db=# select pg_relation_size('log');  pg_relation_size   ------------------                8192  (1 row)


What I would like to do is 

If table log, is greater than xxxx bytes  then truncate table log


Maybe I'll dump it to a flat file first, (yes, it should really be a
flat file to start with, and not in a db.)

Thank you.