YOU CAN MODEL THE EFFECTS OF A NUCLEAR DETONATION OF VARIOUS SIZES with the NukeMap tool.
NYC would suffer, at least, a quarter million dead with the 15 kiloton bomb which was dropped on Hiroshima. This would be the "best case scenario" if someone ever detonated a nuke there. Why? Because almost every nuke designed today is of higher yield.
Worst case? 100 megatons. 8 million dead. 6.6 million injured.