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.
Here is another way to check the size limit:
execute this command at SSH
exim -bP | grep message_size_limit