I'm gonna hafta make a few assumptions for this one. Like how I'll assume the explosion covered the entire door.
The building in the background is 130 pixels.
For two ends, I'll assume a normal skyscraper (150m) and a Supertall Skyscraper (300m)
Low End: 150/130 = 1.1538461538461538 meters per pixel
High-End: 300/130 = 2.3076923076923077 meters per pixel
The diameter is 417 pixels
Low-End: 417*1.1538461538461538/2 = 240.5769230769230673 meters in radius
High-End: 417*2.3076923076923077/2 = 481.15384615384615545 meters in radius
Formula time
Low-End: ((0.24057692307692307709/0.28)^3)/1000 = 0.00063428991183882723193 megatons or 634 tons divided by two is 317 tons or Multi-City Block level
High-End: ((0.48115384615384615419/0.28)^3)/1000 = 0.00507431929471061785578 megatons or 5 kilotons, divided by two is 2.5 kilotons or Small Town level
For which end I prefer? High-End. It's Champion City not Champion City Block.
Version 2[]
So I went up to see if I could get a better pic, let's assume Romeo blew the whole thing up (I mean they stated it a couple times)
The building is 264 pixels
Low-End: 150/264 = 0.5681818181818182 meters per pixel
High-End: 300/264 = 1.1363636363636364 meters per pixel
Size of Champion City: 880 pixels
Low-End: 0.5681818181818182*880/2 = 250 meters exactly for the radius
High-End: 1.1363636363636364*880/2 = 500 meters for the radius
Formula
Low-End: ((0.25/0.28)^3)/1000 = 0.0007117802478134 megatons or 711 tons divided by two is 355 tons or Multi-City Block level
High-End: ((0.5/0.28)^3)/1000 = 0.0056942419825073 or 5.7 kilotons divided by 2 is 2.85 kilotons or Small Town level
Still prefer the High-End but I'm open to other suggestions.