To get at some kind of estimate, let's figure that the world has been producing gold at 50 million ounces a year for 200 years. That number is probably a little high, but when you figure that the Aztecs and the Egyptians produced a fair amount of gold for a long time, it's probably not too far off. Fifty million ounces * 200 years = 10 billion ounces.
Paragraph above came from a Google.com search. 10 billion times $300 = $3 trillion. And not all of it is for sale.
JP Morgan has over $23 trillion in total derivative exposure. They are leveraged 700-1 against their equity. I'm not sure how much of that is gold, but their gold exposure is many times the value of the total amount of gold in existence in the world. That's the game they've been playing over the years, and it has caught up with them.
I've read that it would make a cube roughly 50 feet on a side.
If you would take the time to understand what derivatives are and how they are leveraged against a commodity, you might begin to understand the dimensions of the problem.
LTCM almost broke the financial system of the world and their derivative exposure was magnitudes less than that of either JP Morgan, Citibank, and/or Bank of America all of whom have significantly higher derivative exposures.