In a memoir by a weapons physicist, I read that several hundred megatons is actually the largest possible size, because if you try to chain too many fusion stages together the light from the first stage blows the later stages apart before they can actually ignite. However, I know that Teller, among others, did investigate multi-gigaton-range weapons, so I'm not sure what to make of that. If there really is an upper limit on yield, it's probably classified.
This is fairly correct, as the explosive yield is increased, the shorter the overall time for the reaction to blow apart the reactants. This doesn't have 'a tipping point' just like it doesn't in star cores/fusing layers, but it exponentially decreases the reaction rate for additional energy. Effectively you are reaching the notion of hydrostatic equilibrium in the weapon core for the moment(s) of detonation.
In principle increasing the surrounding pressure on the fireball would allow you to fuse material for longer and build a larger explosion, hence an underground detonation may get you further for the same size of device, but not much since the ground itself will be vaporised too.
Yet if you got the reaction yield up high enough you may be able to unlock higher fusion reaction chains, which, while less energy is realised in each fusion, the total power output of the device is an additive sum of reactions, as each would occur in their own layers.
Much like a hydrogen bomb is hydrogen at the centre of a normal fission bomb such hypothetical 'Nova Bombs' are Fission Bombs around Hydrogen, around attenuation materials, around a reactant core. Such devices* may be able to briefly access a quark-gluon plasma at their cores** which could possibly give rise to the conditions in the outer layers of a supernova. Suffice to say, a very big explosion.
Possible to design and build? We could likely do this. Would we want to? We don't need such a large device, the planet could be 'destroyed' with far less. Yet if we might talk hypothetical, such a device could be an ultimate beacon of 'human civilisation' that could briefly shine across the larger part of our galaxy and even carry a densely encoded message to every star and planet. While it wouldn't be visible to the naked eye like an actual supernova, the point is that a technologically advanced species worthy of understanding the significance of the message will have the telescopes able to see the message, and unlike SETI searching in the radio background for whispers, this would be like using a megaphone and spotlight to say 'we are here'...
...Or rather 'not here' anymore...
*weapon is a fairly useless concept here
**Via channelling energy to a core via attenuation materials.
