The feat in question appears here.
As discovered by Dark-Carioca here, the Nemesis is 1405.7647 meters long. The Nemesis in the image is 297 px long and the blue line explosion is 1104 px long.
1405.7647/297=4.73321447811 m/px
1104x4.73321447811=5225.46878383 m
By plugging that value into this formula for overpressure explosions, W = R^3*((27136*P+8649)^(1/2)/13568-93/13568)^2, W being the yield in tons, R being the radius in m, and P being the pressure in bars (1.37895) we get W=2612.73439191^3*((27136*1.37895+8649)^(1/2)/13568-93/13568)^2. Plugging that into basically any calculator, we get:
W=1433412.87586 tons (1.4 megatons Small City level)
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So here's the calc using a fireball calc. The orange line explosion is about 1069 px.
1069x4.73321447811=5059.8062771 m or 5.0598062771 km
Y=((x/0.28)^3)/1000 with Y in megatons of TNT and x the radius in km. Putting the radius into the calc, we get Y=((2.52990313855/0.28)^3)/1000 and plugging that into a calculator we get:
Y=0.73762832861 megatons (737628 tons/737 kilotons Large Town level+)