It's enough to imagine the political map of South America to guess the right answer: Brazil encompasses almost a half of the whole continent! Thus, it's the largest country of South America.

Specifically, Brazil is bound by the Atlantic Ocean on the east, and it has a coastline of 7,491 km or 4,655 miles. It borders all other South American countries except Ecuador and Chile. It actually occupies 47.3 percent of South America.

More Info: en.wikipedia.org