I'm trying to locate mine ( need it soon to start a batch of warbow arrows), I haven't used it for a while and it's probably in the shed(dark now here), I would think it's around 2.5-3mm mark as it files a groove a little narrower than my round rat tail file. I don't make strings less than 16 strand ff for warbows, which I did not consider, and I always had to widen the nock with the sand cloth stuck on a ruler, cuts out the initial slot nice and speedy. Maybe there is small diameters that can be ordered online?( I'll have a look myself a bit later bearing in mind I'm U.K)
Ruddy Darter.