Even a small nuke will generate a substantial EMP pulse. Everything I've read indicates that an EMP attack is what Iran has in mind. It doesn't even have to hit a target, just explode high in the atmosphere.
WND has run several articles on this threat and it's a very realistic assessment. Iran has curiously launched missiles to an altitude of about 400 km and had them self-destruct, afterwards calling the test a success. A nuclear detonation at that altitude would basically send a powerful EMP burst about 3000 miles in all directions, frying electrical grids, computers, electronic ignitions, cell phones, and other unprotected devices, rendering them useless. Iran has also practiced launching missiles from ships. Even an old SCUD would be sufficient to launch such an attack. It's cheap, effective, and would be almost impossible to stop.
DOD has hardened systems, but civilian infrastructure doesn't. Imagine setting the US back to about 1850 in terms of ability to generate and distribute electricity. The biggest problem we'll have, regardless of who initiates the attack, is maintaining internal order.
Critics of the Strategic Defense Initiative ignorantly bash the concept of shooting down hundreds of incoming ICBMs. The realistic concept is that we really need to stop just one from coming in from a rogue state like Iran or North Korea.
Iran believes, perhaps not without cause, that one EMP burst would cause enough havoc that America would collapse from internal conflict in the aftermath. Just take what happened after Katrina and multiply it to a national scale and it might be a fatal blow. America would still exist, but it would be so torn apart from internal strife that it would cease to effectively act as a superpower. That's why Russia and China are so slow to get on our side on this problem. They see a window of opportunity to lose their chief competitor without having to get involved directly.
It is MUCH easier to make a large (i.e., heavy) nuke than a small nuke.