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.