A thermonuclear bomb is based on fusion of atoms. Light atoms is smashed together. The most common is Deuterium (heavy hydrogen) and Tritium (radioactive hydrogen). When they are fusioned it realese a massive energy.
To start the fusion, it requires a lot of energy. A fission bomb is used as a "detonater". Here is a simple drawing of a fusion bomb.
The upper part is the fission-device, which triggs the fusion-part. Theoretical a thermonuclear bomb can be as big as preferably. A common warhead is W88, used in the SLBM (Submarine Launched Ballistic Missile). It has a yield around 475 kt.
The biggest bomb ever detonated is "Tsar Bomba", which yield was about 50 Mt. Is was tested by the Soviet Union 1961. The biggest bomb tested by USA was Castle Bravo with a yield of 15 Mt. Se more att wikipedia; http://en.wikipedia.org/wiki/Nuclear_weapon_yield The "Tsar Bomba" was first designed to be 100 Mt, if the outer shell was made of Uraniun, so it would be as fission-fusion-fission. But that had causted a large amount of radioactive fallout. So the shell was made of Lead.
ICBM (InterContinental Ballistic Missile) is plased in missile-silos or on a wehicle. Russia has Missiles in the megaton class, USA has Minuteman III, which has been dismantled to one warhead. But the submarines has up to 8 warheads in one missile. Se the list of weapons;