The four story building is 12 meters in height, which is 145 pixels.
0.08 meters per pixel
For the head I'll use sphere calculator.
r = 55 pixels
Volume = 356.8 meters3
For the neck I'll use cylinder volume calculator.
h = 38 pixels
r = 30 pixels
Volume = 54.2 meters3
For the arms I'll use cylinder volume calculator.
h = 442 pixels
r = 61 pixels
Volume = 2555 (5110 for both arms) meters3
For the ribcage I'll use sphere calculator.
r = 102 pixels
Volume = 2226 meters3
For the lower part of the body I'll use cylinder calculator.
h = 82 pixels
r = 45 pixels
Volume = 264.6 meters3
And last but not least - the legs, for which I'll use yet again cylinder calculator.
h = 340
r = 63
Volume = 2136.2 (4272.4 for both legs) meters3
Collective volume is 12284 meters3
Now, for the mass I'll use human density, which is about 985 kg/m^
Demon King's mass = 12099740 kilograms
As for velocity, Meneldor is as fast as Will (228.8 m/s, Subsonic+) and Demon King should definitely be at least as fast as Meneldor (Meneldor himself mentions he is nowhere near Will's level). But to avoid calc stacking, I'll use baseline Subsonic+ speed.
KE = 0.5*12099740*154.35^2 = 34.45 tons of tnt, City-Block level