I pulled apart the extruder today to take a look inside and I also made a few measurements of the extruded filament. The filament has swelled a little, but not as much as I thought. The die diameter is 2mm and at its largest the filament is just over 3mm. Oddly though, the filament is slightly oval shaped, which is either due to a misshaped hole or uneven heating of the barrel.
Also of interest is that the majority of granules that was still in the barrel were not melted and thus able to be poured out. Although keep in mind that this has still only been used just the once.
This leaves me to wonder if the barrel could be reduced in size by a large amount and still extrude. I have a few smaller auger bits around and may try making a mini granular extruder once I have the AC motor geared down reliably.