It's not simple; with large bombs the lethal blast radius grows faster than the lethal prompt radiation radius, but the thermal damage grows fast too. You can duck and cover to survive flying glass and falling roofs, but if you're in line of the thermal pulse you're cooked.

In any case, massive bombs being used in "counter-value attacks" (euphemism for targetting civilians) aren't the most likely way for a nuclear war to start. Both the Soviet Union/Russia and America invested heavily in tactical nuclear weapons, designed for battlefield use (e.g. "counter force".) Additionally, other nuclear powers like France have counter-force first strike as part of their nuclear doctrine. Any semi-rational way to start a nuclear war would be to target the oppositions ability to respond in kind, not to attack cities right off the bat. Strikes against cities are a reaction, a threat meant to deter, but starting the war out that way is just an elaborate form of suicide. Point is, it's a good idea to assume that civilians will have at least some period of disturbing escalation as warning to get the fuck out of urban areas. When you hear about the US military setting off nukes in the Fulga Gap to stop Soviet tanks, that's when you GTFO.