1 Answer

The tiny fonts that are so common make it possible to pack a lot of information it very little space. Unfortunately, this downsized information is often difficult to decipher and digest.
For plain text emails at least, Yahoo! Mail and Yahoo! Mail Classic can make the display bigger by default and allow for easier reading.

Make Yahoo! Mail Display Messages in a Larger Font

To read plain text emails in a larger font in Yahoo! Mail (rich-text messages may define their own fonts and font sizes):

Select Options | Mail Options from the top Yahoo! Mail navigation bar.
Go to the General category.
Set the desired font size (and, optionally, font) under Plain text font:.
Do take a look at the Sample Text. Some fonts appear smaller at a given size than others.
Click Save Changes.
Now click Back to Mail.
Make Yahoo! Mail Classic Display Messages in a Larger Font

To make Yahoo! Mail show plain text messages (HTML messages may define their own fonts and font sizes) in a larger font:

Select Options in Yahoo! Mail.
Go to the General Preferences category.
Make sure Larger is selected under Font Size:.
Click Save.