So, at the end of the Kenichi manga, the elder "sacrifices" his life to take on a missile...and comes back without even a scratch.
So let's estimate how powerful this thing was.
Scaling 1:
Hayato = 2m (at least) = 81 pixels
Rocket width = 288 pixels = 7.11 m
Scaling 2:
Rocket trail (should be approximately equal to rocket width) = 7.11 m = 11 pixels
Explosion width = 601 pixels = 388.5 m
Plugging into the nuke calculator, we get the fireball at the correct range with a yield of 0.082 megatons of TNT or 82 kilotons of TNT
Halving because we are using the nuclear explosion calc, the true value should be: 42 kilotons of TNT or Town level.
Edit:
If near-total fatalities is preferred instead: Air blast radius (near-total fatalities) Yield: Y=((x/0.28)^3)/1000
Y = ((388.25/(2*280))^3)/1000 = 333 tons of TNT, which is halved to be 166 tons of TNT or low Multi-City Block level