The one big problem with your question is this~
Brazil is the largest country in South America,so we would have to know just WHERE these Olympics were to be sited.
Much of Brazil is rain forest,and most of the rest is Amazonian floodplain.
If you were to say where the Olympics might be we might be able to answer you.
There ARE quite a few large cities in Brazil, but as the furthest apart are about 4 hours by plane(from each other) you can see how large a country it is.