How to limit log file size in /var/log/DiagnosticMessages?

While I suspect that this is not what you want to do, you could always do something like schedule a maintenance task to run to limit the size of the file.

Something like this in a shell script that you schedule:

tail -n 10000 /var/log/DiagnosticMessages > /var/log/DiagnosticMessages

Or if you want to keep a history of the contents, something like this:

mv /var/log/DiagnosticMessages /var/log/DiagnosticMessages_$(date +%y%m%d)

If you want to get fancy, you could:

cat /var/log/DiagnosticMessages | wc -l

and then only rename the file if > 10K. As usual it all depends on how much effort you want to put into it, and whether or not you want a history of this file.

And as you said....this doesn't actually resolve the root cause.