How to Configure Max Email Attachment Size ?

There’s no such option in exim as determining the size of an attachment means converting back from BASE64 to the original file format. However, you can restrict the maximum size of a message using:

message_size_limit = 5M

That will put a 5MB message size limit on email if you put the line in the first textarea of the Advanced Mode editor in WHM > Exim Configuration Editor.

or

Here is another way to check the size limit:

execute this command at SSH

exim -bP | grep message_size_limit