Don't forget fallout
Other than direct blast damage (which I'm pretty sure would cover more than 10sq miles), don't forget damage done by fallout. I know that against the USSR we were going to maximize damage to military targets and leave most civilian targets alone (except, perhaps, for military/civilian targets like Moscow), but this is a different situation.
Famous last RPG quotes: "I'll just shoot this fireball down the dungeon passageway..."