How do I send a large file through yahoo mail?

Clarification question - are you asking about Yahoo mail (in your subject) or iCloud mail? If iCloud mail, go Mail/Preferences/Accounts, select your iCloud account, then the Advanced tab, and check Send large attachments with Mail Drop. Then compose the e-mail, add the attachment using the paperclip, and send.

To do so without iCloud, use the Dropbox link for your file. Email the link. This way is easier for large files.