How much reduction of insolation (solar energy) would be required to stop global warming?
It's not that hard to make an estimate.

CO2 traps about 2 watts per square meter.   Direct sunlight at 1 Astronomical Unit is about 1,360 watts per square meter, but spread out over the earth, average night and day it's about 1/4th of that about 340 watts per square meter.  (note, Casey's point is valid, it's probably better to use the number of watts that hits the Earth, so, my estimate is likely a bit low)

There may be more detailed and specific ways to look at it, but this is probably pretty close.   If you shade enough sun to reduce 2 watts out of 340, or 1/170th, that should balance out climate change, at least at current levels.

From space, you can view the earth as a disk in terms of area needed for shade, so the 2D area of the earth is about 127 trillion square meters, and 1/170th of that, about 750 billion square meters, or 750,000 square KM, slightly larger than Texas and as you get closer to the sun, the area needed is reduced by the square of the relative distance.

Still, that's no easy task, to put up a shade the size of Texas into space and it's no easy task to keep it there as it would in effect by a huge solar sale and likely enormously heavy.   It's far easier to simply build mirrors on earth than try to build something in space and I've seen that discussed as a possible solution to climate change.   of-course, they'd need to be kept clean to be effective, it would require maintenance, but it's probably doable.

Source (1,360 watts):  http://earthobservatory.nasa.gov/Features/EnergyBalance/page2.php

sources, 1.5-2 watts per square meter, CO2  (I went with the 2 watts estimate cause that includes CH4).   http://www.easterbrook.ca/steve/2012/01/how-much-extra-energy-are-we-adding-to-the-earth-system/ and https://en.wikipedia.org/wiki/Radiative_forcing