Truncate log lines

Hi,

I'm trying to tune a little bit postgresql logging parameters. I have
a doubt about the lines that are logged. I have some automatic logs
parsing in order to extract some information.

I'm logging some queries that take more than n seconds but I don't
really need to have the entire query logged with this duration is
exceeded.

The problem I have is that a very large queries serie can take a big
amount of disk space. The ideal possibility for my case would be
truncating the log line based on a bytes or number of characters
size.

I've not found any option to do this. My second option was trying to
do this in rsyslog when postgresql-to-syslog is enabled but I've not
been able to do it either.

Have anybody has this problem? Any suggestion to solve the problem?

Thanks