AFAIK, you can just pack as much fuel as you want into the secondary to scale the size of a thermonuclear bomb. So yeah, there is no size limit. Interesting aside; the US had the dial-a-yield mechanism that allowed one bomb to deliver a selectable amount of energy, not sure what the mechanism for this was, however.

As to your second sentence, pretty sure that’s what a MIRV is.

> not sure what the mechanism for this was, however.

Boosting (injection of deuterium/tritium into the centre of the pit) causes a large increase in yield because fusion generates lots of high energy neutrons that go on to fast fission your (still compressed) pit.

    According to one weapons designer, boosting is mainly responsible for the remarkable 100-fold increase in the efficiency of fission weapons since 1945.

Variable yield weapons presumably still exist. Particularly depth charges iirc.

Usually the yield can be selected between two or more fixed levels rather than a continuously variable input. Often this is implemented with tricks such as disabling the secondary and/or disabling boosting in the primary.