Is the Atlantic Ocean growing or shrinking?

Atlantic Ocean growing by a few centimeters every year because of sea floor spreading. Seafloor spreading occurs at divergent plate boundaries as the tectonic plates slowly move away from each other. The Mid-Atlantic Ridge is formed in the Atlantic ocean because of sea floor spreading. It separates the North American plate from the Eurasian plate, and the South American plate from the African plate. 

Further Reading:

• Geomorphic Processes
• The Origin And The Evolution Of The Earth