Length wise:
White Sea – Baltic Canal [227 km] (Opened in 1933, is partly a canalised river, partly an artificial canal, and partly some natural lakes & Shallow depth limits modern vessels from using the canal.)
Dimension Wise:
Panama Canal - [320m x 33.53m x 25.9 m] (Opened in 1914. Links the Caribbean Sea to the Pacific Ocean, creating a shortcut.)