Message size limit
Emails sent from Yahoo Mail accounts cannot be more than 25 megabytes in size.
Whether you have a free Yahoo Mail account or a Mail Plus account, you can send and receive emails up to 25MB in size.
This means that the total encoded size of the email message you're sending cannot exceed 25MB. Attachments sent with emails have to be encoded into a format other email programs can read. This encoding adds about 33% to the original size of the attachment. So a 1.5MB attachment will turn into a 2MB attachment after encoding.
Keeping your emails, attachments and all, under 25MB will help ensure that your emails get where you want them to go quickly.