How To Set The macOS Dock Size From The CLI

Open a terminal and execute the following, replacing 64 with your desired dock size:

defaults write com.apple.dock tilesize -int 64; killall Dock

To lock this dock size, execute the following:

defaults write com.apple.dock size-immutable -bool yes

And to unlock this dock size, execute the following:

defaults write com.apple.dock size-immutable -bool no

All together:

defaults write com.apple.dock size-immutable -bool no
defaults write com.apple.dock tilesize -int 64
killall Dock
defaults write com.apple.dock size-immutable -bool yes