Something requested here, so I'll just make this
Tokyo is 2,190 Square Kilometers, the square root of that is 46.78183358136288 Kilometers
((46.78183358136288/.28)^3)/2 for non nuclear and double for nuclear is...
Non Nuclear: 2331995.0774 Kilotons, 2.3319950774 Gigatons, Large Mountain level+
Nuclear: 4663990.1548 Kilotons, 4.6639901548 Gigatons, Island level