Cobalt bombs are the currently most effective way I know of to make large areas of land unlivable for a long period of time. I don't actually know the fallout area of such a bomb. The Wikipedia article only has the following mildly helpful section:
Theoretically, a device containing 510 metric tons of Co-59 can spread 1 g of the material to each square km of the Earth's surface (510,000,000 km2). If one assumes that all of the material is converted to Co-60 at 100 percent efficiency and if it is spread evenly across the Earth's surface, it is possible for a single bomb to kill every person on Earth. However, in fact, complete 100% conversion into Co-60 is unlikely; a 1957 British experiment at Maralinga showed that Co-59's neutron absorption ability was much lower than predicted, resulting in a very limited formation of Co-60 isotope in practice.
I don't know whether a single bomb that efficiently distributes 510 tons of material is remotely feasible, and the section about Co-60 conversion also suggests that the actual effectiveness of such a bomb would be far below this theoretical maximum.
Does there exist any existing literature that estimates the practical fallout area of a cobalt bomb? Or can we just do a simple Fermi to estimate things ourselves?
I am interested in this question because it seems relevant to estimate the worst case scenario of a nuclear war, and the likelihood of civilization recovering after such an event.
I will award $50 to the best answer by the end of next week, based on my gut sense, and I might pay out more if someone gives a particularly exceptional answer.