nozzle size

The barrel is a 1/2" pipe length/nipple with a machined opening, so a 1/2" plug should do the trick, but you shouldn't expect the same strength as the original material. There a literal fibers of carbon in the original filament, which are small enough to pass through your printer's nozzle but long enough to add strength. Processing the scrap material to go into a Filastruder most likely damages or severs a significant number of these fibers, compromising some of the material's strength. This is in addition to the issue of heat history; common plastics break down slightly each time you heat them beyond a certain point (easily done in the process of 3D-printing).
If you can source a steel plug, and drill a 1.6mm hole to start, then by all means, see how well it extrudes! I don't expect you to cut nozzles in half to inspect for damage the way E3D did, but try the various tips and tricks here on the forums and see how close you can get to usable recycled filament.
As it is, I'm tweaking my own setup so I can experiment with conductive/metallic composites.