Set the desktop composing default
For New Yahoo Mail in a desktop browser, Yahoo documents More options > Settings > Writing email. Under Default rich-text font, choose a style and size. The accompanying preview helps show the result before you use the setting in correspondence.
Enlarge a message on iOS
Yahoo’s mobile help documents an AA control while reading an email on iOS. Open the email, tap AA at the upper right and adjust the slider. This is a reading control; it serves a different purpose from choosing a composing default on the desktop website.
Consider device-wide reading size
Keep email emphasis restrained
Use the main message as the reference when choosing a composing size, rather than only a short preview line. Keep contact details and long URLs readable alongside the paragraph text. A simple hierarchy with a brief heading and ordinary body copy is easier to maintain than a different font for every line.
Your questions, answered.
Will the writing default enlarge emails I receive?
The documented writing preference and reading controls address different tasks. Use a reading control for received mail.
Where is the mobile AA control documented?
Yahoo documents it for reading an email on iOS; do not assume the same control exists in every Yahoo client.
Sources & background
Sources supporting this page’s explanations. Practical examples are our editorial suggestions.