I'm assuming you're on a 32bit OS and hitting the 2GB file limit. At any rate, you don't want your logs to get this big anyway. Exim log rotation is handled by the system logrotate. You should have a config in /etc/logrotate.d/exim. By default it rotates weekly. If that's not happening, you should verify that logrotate is actually running. If it is running, and you are just hitting 2GB within a week, you could modify the logrotate to work by size, or even daily.
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