So, in 2011 the world's humans emitted 31.6 gigatons of carbon dioxide from burning fossil fuels (and probably another 5 Gt or so from land use changes, but I'll ignore that for now).
That's 8.6 billion metric tons of carbon, and it were all in the form of diamond, it would be a cube 1.4 km on a side.
That diamond would weigh over 40,000 trillion carats -- it would take 330 million years to mine that amount, at current mining rates.
It would be worth about $3,000,000 trillion dollars, or over 42,000 times the GDP of the entire world.
It would be a sizeable mountain: as a pyramid whose base is a square, with each side equal to its height, it would be 1,900 meters tall (6,400 ft).
It would be the 1,459 times the size of one of the largest of the World Trade Centers. With the same base area of one of the towers, it would reach 378 miles high, or almost twice as high as the International Space Station.
If it were a rod that reached to the moon, it would have a diameter of over 9 feet.
In other words, it's a lot of CO2! (But if it were all at the surface of the Earth, at sea-level temperature and pressure, it would be a layer only 3.3 cm thick.)
Note: Corrections cheerfully accepted.
Added: Lest these numbers seem too surprising, remember that the Earth's atmosphere weighs about 5 million gigatons (to first-order it's simply PsAs/gs, where Ps is the surface atmospheric pressure, As the area of the Earth's surface, and gs the acceleration due to gravity at the Earth's surface). And 2,000 Gt of that is CO2 (if the CO2 level is 395 ppm).