Could I have some info on what would be realistic ranges for EMPs from nuclear bombs of different yields? I'm unclear about that. Of course it depends on altitude and blast yield, and some electronics can take more than others, but just in general what area would it affect?
Starfish Prime had an EMP blast radius of approximately 900 miles. More if elevation is taken into effect.
This Wiki page shows a map with most of the US affected, which would be approximately 1500-2000 miles.
But of course this all depends on yield, height of detonation, bomb design. The Wiki hints at a super-powerful gamma detonation that would produce a much more intense EMP, but its existence is not confirmed.
You've got a lot of leeway to work with. For a limit, I'd suggest staying within the visual horizon, as suggested but not confirmed in the article linked.