First, Russia is unlikely to nuke anyone, assuming of course that their nukes actually work. The consequences for a nation that did decide to let off the big firecrackers…. Well, no one has a large enough army or enough nuclear weapons to fight the rest of the world. Or rather, what would remain of it, and the Geneva Conventions would become more the Geneva Suggestions.

As for warning time, that depends on the deployment method. Tactical weapons, you wouldn’t have much, if any, warning. Strategic weapons, anywhere from 5 minutes to a few hours, depending. Tactical weapons tend to have ranges somewhere in the 100 km range, and have, generally a much smaller blast radius.

Interceptors have been developed, (s400, Patriot and others) but how well they would work against a nuclear warhead is untested as far as I know. I think it would depend on how the warhead was fused and triggered.

As for your last question, no clue. My understanding that current warheads range in size from something that can take out a city block to weapons that can take out New York City and everything in between. The only thing the average person can really do is treat it like you would a tornado. Head to your nearest shelter and stay away from windows.

Dr. Tyson says that modern nukes don’t usually have much radiation fallout anymore but still, be upwind if you can and take a shower after the attack but don’t use conditioner.

If the thought of possible nuclear war scares you, you might want to consider learning more about it. There are two channels on YouTube I would recommend as a starting off point, Perun and Kyle Hill. Perun does military economic analysis and Kyle Hill is a science educator with an emphasis of nuclear technology. Both have done presentations on nuclear weapons and their use.