The likelihood of a nuclear attack by a terrorist agency is pretty good. Chances are it will be a dirty bomb...no fission or fusion, just a conventional bomb with radioactive materials attached to create local fallout in the blast area and downwind. Pop such a device in a major city and you'll have done some serious damage.
Next order of likelihood is a low yield tactical device. A few kilotons. Messy, but not crippling.
As the yield of the device increases, the chances of detecting it increase thanks to stray radiation, etc. some of which can be tracked from ORBIT. The simple logistical complications also start to mount and you quickly run out of options for delivering the device to the target reliably and without being detected.
So, as far as terrorist devices are concerned, we are probably talking about a fairly small tactical device. We are also not going to be talking about a lot of them. Scraping together the materials for one or two devices is entirely possible, but 100? Yes I know about the reported Soviet backpack nukes. But those aren't cheap either. Terrorists would burn an awful lot of operating capital on just one or two devices (if they were available to them).
So, we are going to see a detonation in a major city center, probably New York or Washington D.C. New York is a higher probability target because it won't be AS closely monitored for this sort of thing as DC would be.
I live well away from any major city. You'd have to drop a 25 megaton device on Worcester, MA for my area to even feel the breeze. That doesn't mean I would feel no effects, but they would mostly be societal and logistical rather than direct damage. (There's not much west of me suitable for terrorist attack either.) I'd be quite a bit more concerned about a full scale war exchange though.
(Cont.)