A ruler would be shown in such a way that you see the markings, so the actual length of the ruler is irrelevant. There's still the problem that the markings could be in centimiters or inches. You can usually decide which it is from the number of subdivisions, but not always. Cds are good for the same reason: even though there are cds of smaller size than the normal, the hole is always the same.
The digs I have been on use a surveyer's ranging pole (red/white bands), which seems to be fairly standard. But it depends on what "small" is: a ranging pole would be no good to show the size of a coin, but a ruler would not be useful to show the size of a small building. Size matters.
Coins are attractive options because they're common and their dimensions are relatively easy to obtain, but they are somewhat restricted to the country the photo is taken in. Rulers at the least are based on international standards of measurement.
Credit cards I'm not sure about, they all appear to be of regular size but I don't know if their dimensions are standardized.
CDs and floppy discs are also creative solutions, they're common enough objects to give a sense of scale, and they have to be of similar enough size to be accepted by the opening on a floppy drive.
Still for sheer precision and versatility, I have to choose the ruler.