So I switched to new yahoo mail and still don't see how to enlarge or change fonts. Why do you bury this stuff?
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C. R. commented
in the Compose window, there's icons at the bottom to choose font size etc.. The Aa icon has the style and size choices.
to change account settings, click on the Gear icon in the right panel, scroll down and click 'more settings'. then click 'writing email' in the left pane, and you'll see 'default rich text font' with a FEW choices of font styles and 6 choices of size.
for just viewing inbox emails, you have to adjust your BROWSER settings to increase the font size for the whole page.