With a Free Yahoo! Mail account, you can send and receive messages up to 10MB in size. This means that the total encoded size of the email message you are sending cannot exceed 10.0MB. Attachments sent with emails have to be encoded into a format other email programs can read. This encoding, called base64 encoding, adds about 33% to the original size of the attachment. Thus, a 1.5 megabyte attachment will turn into a 2 megabyte attachment after encoding.