700 miles is too far even with a 20 Megaton warhead, but even if they had one that large, the missile only has a 500 kg payload capacity. This is an order of magnitude lighter than the likely weight of any North Korean nuclear device.
http://en.wikipedia.org/wiki/Taepodong-2The maximum achievable ratio of a fusion bomb is about 5.2 MT/t, so even if loaded with a thermonuclear weapon, the max for a Taepodong-2 would be 2-3 MT. But this is a much more advanced weapon than they are capable of building.
http://en.wikipedia.org/wiki/Nuclear_weapon_yield