You get the "non-zero size" warning if you have an unresolved error reported in paniclog and/or even if you resolved the error, you will continue to receive an alert/email until you hit your logrotate rotation limit, delete the file, or adjust/instruct logrotate to archive it sooner.
Issue #1 - people are responding about particular errors in the paniclog which may or may not be related to the OP's. There are, however, hundreds of issues that can cause the same report.
Issue #2 - regardless of what error is in paniclog, exim will continue to email you (even when resolved / fixed) until the file rotates or is deleted.
It is important to disambiguate between particular misconfigurations causing the report / paniclog, and continuing to receive the report despite resolving the issue.
There is no harm in deleting the paniclog file. Exim will re-create the file if there is a new issue, or if you never fixed the initial issue. However, I agree with @Jackfritt that the proper way to address this is with logrotate, as that will help one debug the initial issue causing the report. However, solving that issue will not stop exim or its mail agent from continuing to send the report; that requires logrotate or deletion.