Late reply but my original numbers were wrong, it’s 7E27 atoms in a human according to wolfram, I think I just misinterpreted its original output when I tried to put it all in one formula: https://www.wolframalpha.com/input?i=how+many+atoms+in+a+hum...

The answer is still close to 60 LY (73.99LY) but it changes the cube size significantly:

https://www.wolframalpha.com/input?i=%288e9*7e27%29%5E%281%2...

(8e9*7e27)^(1/3)*1e-10 = ~382 meters on each side of the cube.