You can watch the asteroid's fly by on Slooh.com this evening.
The asteroid is relatively large in terms of asteroid size, roughly measuring between 1,000 and 2,300 feet across. That would be the equivalent of between two and 4.6 city halls (including the Billy Penn tower).
2012 LZ1 is large enough to cause serious damage should it have impacted Earth (again, it won't) but it's certainly not large enough to wipe out human civilization as we know it. In fact, it will likely not be visible to the naked eye this evening towards sunset as its brightness magnitude is significantly lower than that of Venus. It will be close enough, however, for astronomers to chalk this up as another "near Earth" asteroids, of which there are about 9,000.