The Tsar Bomb test of 1961 was over 50 MT, and could have been scaled to 100MT.
Weighing 30 tons, it was too big to be transported by air. An unmanned sub, operating as a long range torpedo, could carry a really, really, really big bomb.
The Tsar Bomba was scaled back because after about 50-60 MT yield the energy produced would be mostly wasted. It was detonated at about 15,000 feet and the fireball touched the ground and the top of the fireball was at the tropopause. A larger bomb could be built but what is the point? It is a very inefficient use of weapon material. There would be little question about who did it until other countries developed the same capability.