How do I change the fonts size or font?

General

Picard is a Qt application, so whatever works for other Qt application will work for Picard.

Linux specific

Two ways to change the size/scaling are:

1. Desktop Settings

   The usual default way to change the scaling is to rely on the settings of the desktop environment, e.g. on GNOME the scaling set when configuring the screens. This usually works, and should be the first approach to take.

2. Environment Settings

   Setting the environment variable QT_SCALE_FACTOR will scale fonts for Qt applications. This can be useful on 4k monitors. From a terminal, you can then run QT_SCALE_FACTOR=2.0 picard to start Picard, or create an alias in your shell, or set the variable in your shell to affect all Qt applications.

To change both the size/scaling and the font, use qt5ct to set the new font and/or size. Note that these changes will apply to all Qt applications, not just Picard.

The official documentation on high DPI could be a useful read as well.