Let's run some numbers on this idea . . .

The solar constant is ~1367 Watts per Meter squared
http://en.wikipedia.org/wiki/Solar_Constant

Global Warming is now about 3 watts per meter squared, 0.2% of the
solar constant.
http://www.giss.nasa.gov/data/simodel/
(It will quadruple in a century or two.)

The cross sectional area of the Earth is 125,000,000 km=B2
http://en.wikipedia.org/wiki/Solar_Constant

The area of Aluminum sequin in orbit needed to remove the current
global warming is, therefore roughly .002 * 125,000,000 km squared or
300000 km squared.

If your sequins were one 10,000th of a meter thick, you would need to
orbit 30 km cubed worth of Aluminum.

The density of Aluminum is 2.70 g/cm=B3
http://en.wikipedia.org/wiki/Aluminum

one km cubed is 10 to the 15 cm cubed.

So you want to orbit 8 times 10 to the 13 the power kg of Aluminum.

Well then, GOOD LUCK.
Even if you could reduce this by, say, making your sequins
1/1000 thiner, it would still be impossible. But . . .

I've got an idea! WHY NOT JUST REDUCE THE EMISSIONS
OF GREENHOUSE GASES? Wouldn't that be easier?
Of course, one could not write good science fiction, because
this solution is too practical.