How do I reset to default window sizes in MacOS Safari's Responsive Design Mode?

Solution 1:

.This setting is written to Safari's preferences, that are located in ~/Library/Preferences/com.apple.Safari.plist in dictionary by ResponsiveDesignCustomPresetConfigurations key. So if you need to reset all responsive mode settings - just close Safari and delete that dictionary from prefs .plist file with following terminal command:

defaults delete ~/Library/Preferences/com.apple.Safari.plist ResponsiveDesignCustomPresetConfigurations

Solution 2:

The responsive design mode can be reset by removing the ResponsiveDesignCustomPresetConfigurations in com.apple.Safari.plist. The plist file location depends on the versio OS.

macOS 10.15 Catalina (and 10.14)

• Go to System Preferences → Security and Privacy
• Enable Full Disk Access for Terminal
• Run the following command

  defaults delete ~/Library/Containers/com.apple.Safari/Data/Library/Preferences/com.apple.Safari.plist ResponsiveDesignCustomPresetConfigurations
  

If full disk access is not enabled, the plist file cannot be found, read or written to.

macOS 10.13 and earlier

Editing the file can be done using the plist file in: ~/Library/Preferences/com.apple.Safari.plist