Well, we use 7,300,000,000 barrels of oil per year, which ends up being about half of our total energy consumption. Therefore, I would estimate that we'd need 2.35 million square miles to replace oil.

I read that 67% of the oil is used for transportation.

It's pretty clear to me that corn ethanol is a complete joke, and even the most efficient forms are extremely questionable. Pesticides, fertilizers (petroleum based), soil erosion etc. complicate matters further.

Back to corn, yeah, let's convert all spare land to corn and have filthy farm equipment polluting the air so we can get a whopping 10% energy gain. Completely ridiculous.