Set to an Integer Value (default is 64)
Dock icon size of 32 pixels.
defaults write com.apple.dock "tilesize" -int "32" && killall Dock
Dock icon size of 48 pixels.
defaults write com.apple.dock "tilesize" -int "48" && killall Dock
Read Current Value
defaults read com.apple.dock "tilesize"
Reset to Default
defaults delete com.apple.dock "tilesize" && killall Dock