Sorry, coming a bit late to this, but my rough guide to files is:
Saw files are usually termed 6 sided, even though they are triangular. This is because of the three flat sides, plus 3 slightly curved intersections. It is this bit that cuts the depth of the gullet and usually wears out or fragments first as all the stress is presented here. When this wears out, the file stops cutting even though the sides are in good order.
Look for a dimensioned cross section of the file; to get the right size for your size and number of teeth per saw, the width of each flat side should be a small amount greater than twice the depth of the tooth to the gullet. The radius of the intersection at this size should be correct for the shape and size of your tooth. This means that you have three pairs of cutting sides per file as you rotate it, leaving the minimum amount of unused file teeth in the middle of each face. I always try to use the faces in sequence so that I can control the wear of the file.
A good idea if you are deep-cutting teeth to a new shape is to slightly undercut the base of the gullet with a hack saw to minimise wear on the vulnerable part of the file..... they last longer this way.
2 xx slim, about 4 “ long should be OK for a16 pointand is about as small as so-called saw-files get. Increases in the number of teeth means you’ll probably need needle files, but they tend to be a bit delicate and wear quickly. Avoid cheap ones and try to match the shape to what I described above, especially a row of teeth that cut in the gullet. If the file is blind on the corners you cannot cut the gullet, however small the file and the file won't then cut the sides.
Best brand? Hard to say as saw files are getting cheapened out of sight and the quality is often poor. What was once a good make may not still be the case. I favour Bahco, but that’s me. Expect to pay about £4 - £5 for a good 5 inch file.
We all have our way of doing things, hope that this helps,
