Files are sized by length and by their thickness (shorter files are naturally thinner to start with). Then, there is a variety of sizes available in each length... This is what follows after the length, e.g. a 6" 'Slim Taper'.
The common size tapered saw files you will find are 4" through 8" long, and are listed as (in order from widest to narrowest):
Extra Slim Taper (aka Xtra slim or X slim taper)
Double Extra Slim Taper (aka XXtra Slim or 2x slim taper)
The "taper" is just that, a slight taper to the thickness of the file at the end. It aids in starting the file to cut properly and evenly.
Here's a quick visual on some of the different sizes - these are essentially the file sizes I keep on hand for my own use:
Smaller files (and the smaller as listed above) have a sharper corner, therefore leave a longer tooth (and a deeper gullet) when you sharpen. This is important in fine-toothed saws, especially those 12 ppi and finer, as the gullet carries away the sawdust... Too round of a corner can also make sharpening more difficult, as the file is just too big to fit in the slot for the tooth. However - too sharp of a gullet can be a problem too, making it easier for the metal to tear at the base of the gullet... That's usually not a big issue, but it's something that Grimshaw thought important enough to mention, so I won't question his authority - and I have seen it, though admittedly only on saws with poor quality steel.
Finding a balance is what it's all about. The file needs to be properly sized for the tooth you are working on - too small and you risk wasting your file quickly. Too big and it' also a waste:
Note that I did not illustrate a rounded corner in the section through the file in the illustration above - it comes to a sharp point, which is not the way it is in reality. It is slightly rounded over, and that is the roundness referred to above. Larger files will have a larger radius at the corners.
Not all of the classic texts nor current web sites agree on the exact size you should use for a given PPI... This may have been because of what was available or perhaps even what a particular company sold or had on hand... Here's the table I put together from a plethora of different sources:
Simonds (a well known maker of saw files) recommends these sizes:
Tools for Working Wood recommends
the following sizes:
Needle File for 15ppi and finer saws ( file is 7 3/4" long x 2nd cut)
4" Double Extra Slim (12-15 ppi - short stroke)
4" Extra Slim (11-13 ppi)
5" Extra Slim (11-12 ppi)
6" Double Extra Slim (10-11 ppi)
6" Extra Slim (9-10 ppi)
7" Extra Slim (8-9 ppi)
7" Slim (5-7ppi)
You can see it varies from what I put together, and it's just more evidence there is no hard fast rule. There is no reason to doubt their recommendations - they have a pretty good grasp of what saws require, and following their advice won't steer you wrong.
For reference - the difference between TPI (Teeth Per Inch) and PPI (Points Per Inch) is just in where you measure the tooth - from point to point or from gullet to gullet. PPI is point of tooth to point of tooth. Some text refer to TPI, while others refer to PPI, so I've included both in the table above.
I hope that helps clear things up a bit... I'm sure it's way more than you wanted to hear...
The Art of Saw-Filing: Scientifically Treated and Explained on Philosophical Principles By Henry Wells Holly
First published in 1864, still in publication
Available at Google books here:
You can download a PDF copy from Norse Woodsmith here:
Grimshaw on Saws
Available on Google books here: