Yeah I thought that was an interesting thing to bring up at the end of the review. Personally I thought it was 1.75mm.
Out of interest it says on the PP3DP site 1.73mm. On the 3DPrintingsystems site it says 1.75mm. I tried to measure it with my verniers but its really hard to get reliable, repeatable measurements. At the end of the day I don't think it really matters much as most filament, even the UP stuff, varies a little in size. I'm not sure what the threshold is, and it probably depends a little on the material's melting point as well, but provided its not consistently ~1.6mm or similar it likely wouldn't be a problem. I don't think an increase in size, within reason of course, is an issue. YMMV.