Through the system preferences you can adjust the size of the Dock icons. Unfortunately, you can’t go larger than 256 pixels. With the following command you can change the size in different sizes.
defaults write com.apple.dock largesize -float SIZE
Change SIZE in a desired number and restart the Dock with the following command
Restore the default setting or change the size in System Preferences (Dock)
defaults write com.apple.dock largesize -float 128