KSEF-SIZE
A check of the Polish KSeF FA(3) system – applies to invoices submitted to KSeF; a separate format, not Peppol.
What this error means
KSEF-SIZE
The file exceeds 1 MB. KSeF rejects invoices larger than 1 MB (without attachments).
How to fix it
Remove embedded content and attachments from the XML — a KSeF invoice must not carry attachments inside the file.