Hopefully you have a timestamp column in your table (or have the freedom to add one). AFAIK, you have to add the timestamp explicitly when you add data to the table. Then you can do something along the lines of:
DELETE FROM tablename WHERE timestamp < '<a date two days in the past, or whatever'>;
You'd probably want to just do this periodically, rather than every time you add to the table.
I suppose you could also just limit the size to the most recent ten records by checking the size of the table every time you are about to add a line, and deleting the oldest record (again, using the timestamp column you added) if adding the new record will make it too large.
Falkon's answer is good - though you might not want to have your archive in a table, depending on your needs for that forensic data. You could also set up a cron job that just uses mysqldump to make a backup of the database (with the date in the filename), and then delete the excess records. This way you can easily make backups of your old data, or search it with whatever tool, and your database stays small.