Settings/PreferencesAppearance & Behavior | Appearance- Enable
Use custom fontoption — it will allow to change font used for GUI as well override its size.

In older (2018.2 and older) that option was named Override default fonts by:

Settings/PreferencesAppearance & Behavior | AppearanceUse custom font option — it will allow to change font used for GUI as well override its size.
In older (2018.2 and older) that option was named Override default fonts by:
