I had to constrain my estimate somehow. The plasma has to be able to fit in a thin tube, probably built by hand. The electrode gap is adjustable by means of a large lever, which should imply a limit to the internal pressures/vacuums feasible.
Z-pinch tubes are cylindrical tubes made of quartz (or similar) that contain a plasma in which a high current is passed through to compress it. In other words, the Quark X is a z-pinch tube almost by definition. In the 1950s most tubes had a diameter of a few cms but then it was shown that a smaller diameter was beneficial to reach higher plasma densities when compressed. Interestingly enough, the Quark X has some dimensions that are compatible with the criteria known to lead to enhanced plasma stability.
No neutrons, etc. seem to be coming out of the tube.
Do we know if Rossi has ever worked with deuterium?
My general impression was that simply using the Stefan-Boltzmann equation for a plasma, which is considered to be a blackbody in this case, within a thin transparent tube, is quite easily subject to errors that can easily span orders of magnitude compared to a solid that is a blackbody of the same dimensions.
If anyone wants to work out the plasma density/pressure/temperature/frequency required to make a blackbody in the space allotted by the Quark design, please have a go at it.
As soon as a current passes through the plasma, magnetic Lorentz forces contract the plasma which forms a filament in the center of the tube with minimum pinch diameter. From what you wrote I get the impression that you would like to assume that the pressure, temperature and density of the plasma remain isotropic within the tube. This cannot be the case precisely because of the pinch. In other words I don't think that a good set of conditions exist "to make a blackbody" and indeed the Stefan-Boltzmann equation not fit for purpose.