A flat file cuts on both faces and both edges, which is what lets it take down a surface and square an internal corner in the same pass. That is the distinction from a hand file, which carries a safe uncut edge precisely so it will not cut the shoulder it is worked against.
Cut grade is the first decision and it is the one that decides how the job runs. Bastard cut removes material fast and leaves a rough surface. Second cut removes moderately and is the general working grade. Smooth cut removes very little and leaves the finish. The grade is stamped on the file rather than judged by looking at the teeth.
Most work uses more than one grade in sequence rather than one file throughout. Cutting down with a bastard file and finishing with a second or smooth cut is faster than working a fine file hard, and it is easier on the file.
Length is chosen against the surface rather than the workpiece, since a short file follows a hollow already in the metal while a longer one bridges it. The sizes here run 12 x 150mm, 12 x 200mm, 12 x 250mm and 6 x 300mm. Files are sold on the tang, so a handle is fitted separately. For an unfamiliar file, send a photo to 07754288583 on WhatsApp. Width is stated alongside length because it decides what the file will enter as much as what it will flatten.