Using classic Outlook for Windows in business environments
To increase the font size in Classic Outlook, you can follow the steps below:
1/ Select the folder where you want to adjust the font size
2/ On the ribbon, select View (or press Alt + V) > Current View (or press Y+1) >View Settings (or press V)
3/ Select Other Settings (or press O)
4/ To change the font or font size for the column headers > select Column Font (or press C)> then make your desired adjustment
5/ To change the font or font size of the message subject lines > select Row Font (or press W) > then apply your preferred settings
For temporarily enlarging an email in the Reading Pane in Classic Outlook, you can try using the zoom option available directly within the Reading Pane.
Reference: Change the font or font size in the message list - Microsoft Support
I hope this helps and provides a clear direction. If you have any updates after trying these steps, please feel free to share.
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