**All Apps**  
Go to Settings Manager > Appearance > Fonts and change the size of the default font.

**Just Thunar**  
Add the following snippet to ~/.config/gtk-3.0/gtk.css (create the file if it doesn't exist):

<code>.thunar { font-size: 13pt }
</code>

To have it take effect you either need to restart thunar and the thunar daemon;

<code>thunar -q && thunar</code>