If they want to launch it at us, they've pretty much got to get it small enough to fit in a car.
Not that it really matters anyway -- the NPRK would only launch a nuclear first strike as a form of ritual suicide. MAD still applies, even to nasty little third-world dictatorships, and launching a single nuclear missile (or even a few of them) makes no sense strategically; in a nuclear war you need to knock out your opponent's nuclear response capability or they're going to respond by nuking you to ashes in short order.
If North Korea did decide to nuke someone, they'd be much better served to smuggle the nuke aboard a ship and detonate it in a harbor somewhere; at least then they'd have some fig leaf of plausible deniability.
Yeah, not so much. Not that the Norks wouldn't try that dodge, but fingerprinting nuclear weapons is a thing. Within hours of it's use, we'll know whose it was, or at least who built it.