One of them, the Bootes Void, for instance, is around 1 billion light years wide. How do they observe them??. From taking the redshifts of the galaxies that lie in that direction and plotting a 3D representation of their distances (see these sites...
2Df Galaxy,
6Df Galaxy,
SDSS). What they've found in the surveys is that the galaxies seem to be mostly concentrated in huge domains called walls....basically multi-megaparsec streams of galaxy superclusters that appear to surround what are basically nearly empty (to our perspective) areas that they call voids. It's a lot like soap bubbles...the walls of galaxies form the bubble and the interior of the bubble is the void. The Bootes Void is one of the largest they've found. Everywhere they've looked, they've seen the same pattern repeat itself over and over again. The maps on the sites I've given you will show you what I mean.