Anyway, here are the size differences from Dow Jones,
The addition of Venezuela gives Mercosur a combined market of 250 million people and a combined output of $1 trillion in goods and services annually, said Brazilian President Luiz Inacio Lula da Silva during Friday's summit. The other members are Paraguay and Uruguay.
NAFTA, combining the markets of the U.S., Canada and Mexico, has 450 million consumers and a combined gross product of about $14 trillion.
http://online.wsj.com/article/BT-CO-20060722-700169-search.html?KEYWORDS=cuba&COLLECTION=autowire/6month