The file size depends on what size chain your saw has on it. If you look at the sides of the chain you will see numbers stamped on the links to indicate the size, or pitch, of the chain.
Generally speaking a 32 pitch chain uses a 3/16" (4.8 mm) file, a 38 pitch uses a 7/32" (5.5 mm), and a 40 or 50 uses a 1/4" (6.4 mm). You may want to check with the chain manufacturer to be sure.
It is possible for the chain teeth to wear to a point where they require a different file size than normal but at that stage a new chain is a good idea anyway as the efficiency will be down and that much wear could make a chain more likely to break.