automated row deletion

I am inserting 250 rows of data (~2kbytes/row) every 5 seconds into a table (the primary key is a big serial). I need to be able to limit the size of the table to prevent filling up the disk. Is there a way to setup the table to do this automatically or do I have to periodically figure out how many rows are in the table and delete the oldest rows manually?

Thanks,
Dave

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