Every web based email provider has different email attachment limits. Users cannot send or receive emails with attachments exceeding that size limit. While Gmail users can send email with attachments upto 25MB, Yahoo Mail has less permittable email attachment size limit.
Email attachment size limit on Yahoo
A free Yahoo Mail users have email attachment limit of 10MB, while paid Yahoo Mail Plus users have double email attachment limit of 20MB.
Since most of email providers have 10MB email attachment limit, it is advisable to keep email attachment size below 10MB for prompt and confirmed message delivery.