This tip is specific to Windows 8. The only way to change the font sizes in Windows 10 is by increasing the display %.
This tip comes from Cowboy:
As we all know, Microsoft removed the ability to change the font and font size in the Outlook 2013 folder list. But all is not lost. For those people with high resolution monitors and/or poor vision there is something we can do short of enlarging the whole display image.
Right click on the Windows 8 desktop and choose Personalize then click Display.
There is an item "Change only text size". If you click on the dropdown you will see the items where you can change the text are Title Bars, Menus, Message Boxes, Palette Titles, Icons and Tool Tips.
You cannot change the font, but you can change the size and make it bold. I found if I just change the menus size from 9 to 10 it makes the folder list text larger. It also increases the size of the ribbon which in my case is fine. One client changed almost everything to 12 which I think is a little over the top, but he really likes it. So if you are having trouble reading text in Office 2013 applications, play around with it and I think you will be able make some adjustments which will really improve the look and feel of outlook and other applications as well.
If the font size in the message list is too large, you can change the size in View Settings, Other Settings and Conditional Formatting.
In these screenshots, I changed the font from the default 9 pt to 10 pt.
Click Window Color and find Menu in the list of Items.
Change the font size tutorial
Published February 14, 2013. Last updated on June 15, 2017.