I have a Lenovo X1 Carbon that has a 14 inch screen with a 2560x1440 resolution. While not quite Retina Display quality, it still has enough pixels to make everything on the screen appear tiny with a regular 96 DPI setting. Trying to fix this in browsers by increasing font sizes breaks quite a few sites, while custom DPI settings works better.
Firefox
Open about:config and set the property layout.css.devPixelsPerPx to the
desired value, in my case 1.3.
Chromium
To change DPI settings in Chromium, start it with
--force-device-scale-factor=<value>. If you don’t want to launch chromium from
the terminal you can edit the .desktop file, on Debian it is located at
/usr/share/applications/chromium.desktop
Locate the line that looks like exec /usr/bin/chromium %U and edit it to look
like exec /usr/bin/chromium --force-device-scale-factor=<value> %U, and you should
be good to go.