Nagasaki was a baby nuke by todays standards. what we would toss today is 100 times more powerfull than that. Same goes for what we might expect to be detonated here. Depends on how much fision material they get their hands on.
Probably not. The biggest weapons that its practical to smuggle in are the aledged ex Soviet Suitcase bombs which are rather small devices (to the extent they exist at all).
Anything bigger would require a ship or plane for delivery, and those are getting pretty close scrutiny these days.