Since Email to Case Premium is native to Salesforce, it relies on Salesforce limits.

 

For emails sent out of the New Comment Page:

Email Messages must be within 25 MB (including Files, Attachments, HTML, and text Body).

Please see the following articles for additional information:

• Attachment size limit for On-Demand Email-to-Case
• No Bounce, Error or NDR for Oversize (25+ MB) Emails

 

For emails sent out of the Reply & Timeline Component in Digital Experience Sites: 

Email Messages must be within 2 GB (including Files, Attachments, HTML, & Text Body).

 

When emails are sent this way, recipients will receive a Content Delivery link to the file via email rather than the file itself. The link should not require Salesforce authentication to be viewed. The file is saved on the Case Record in Salesforce, in the appropriate related list and also displayed in Timeline.