The answer that you have given is correct. Brazil is the country that best fits this definition.
In your question, you say that this country’s economy “outweighs that of all other South American countries.” The most typical way to measure the size of a country’s economy is by looking at its overall level of Gross Domestic Product (GDP) (as opposed to its GDP per capita). Brazil’s GDP is much higher than that of any other South American country. In fact, Brazil’s GDP is almost as large as the GDP of all of the other countries of South America combined.
That leaves the size of the country in question. Of course, Brazil is by far the largest country in South America. It is almost as big as the United States. Brazil’s land area is 3.288 million square miles while that of the US is 3.794 million square miles.
Thus, Brazil is clearly the best answer.