By default Exim maximum message size is 50MB, you can confirm it by typing
# exim -bP | grep message_size_limit message_size_limit = 50M
You can change maximum message size by adding line
message_size_limit = 5M
on the first textarea of the Advanced Mode editor in WHM > Exim Configuration Editor. That will put a 5MB message size limit on email.
PS: the limit only affect is email is sent directly using Email Client, if using Webmail (Squirrelmail) there is another limitation set by PHP when uploading file. it’s variable upload_max_filesize, you can change the value on WHM > PHP Configuration Editor. Restart apache to make sure variable.