I think a nuke launched to explode at apogee 100 miles above NY would take out most of New England and the Mid Atlantic states, probably all the way to Ohio. I'm sure the experts will be along to fill us in.
But if terrorists had one nuke, I think they'd use it to craterize NYC or DC, not explode in space to take out our grid.
I have a copy of Bruce Blair's
Strategic Command and Control open to the EMP diagram on page 91. The scenario is a high-yield thermonuclear detonation 62 miles above the Earth, centered on western Nebraska. It would have EMP'd every Minuteman field we had in the mid-1960s (Minuteman launch control centers were not EMP-hardened until the early 1970s). The radius of the 20,000-volt-per-meter ring is 744 miles (coincidentally, exactly 12 times the burst height of 62 miles).
If the 62-mile figure were used, but the blast hypocenter is NYC, the circle would just about reach the Indiana-Illinois border. A 100-mile burst height would probably generate an EMP ring about 1,200 miles across; this would extend to somewhere around Kansas City.