The truth is that it would take a tremendous number of EMP weapons to put down all of the US. One of these type weapons...might be able to take out the Miami area...but that's it. But in the case of Ossama and the boys...if they could just put one bomb on mid-town Manhattan...the Wall Street crowd would go down...for at least six months while the whole place got rewired. In this scenario...thats the major extent of damage that you could expect.
Scale naturally applies to a significant degree - a 10-20kt yield would do only a fraction of the EMP damage of a 1-2mt yield device detonated at the same altitude, but the latter's damage would not be a simple scaling up - smaller yield devices are relatively more efficient, but how much more so depends on the altitude of detonation. The same destructive effect differences appear in blast effects for atmosphere bursts (effective yield) but the numbers are different. The EMP scaling effects depend so much on detonation altitude that I have no idea what the numbers might be.