Avoid using objects such as coins for scale, as they will require people unfamiliar with them to look up the dimensions or guess, both of which defeat the purpose of the object in the first place. Coins are particularly bad as they can reinforce a geographical bias, and some coins' designs are copyrighted. This is particularly true for coins such as the "dime", which non-Americans would first need to look up how many cents this equates to, then look in a table such as that below.
Ideally, a photograph should include a ruler with the subject (as seen in this photo) or add a scale marking via an image editor instead (as seen in this photo). SI ("metric") units are the most commonly used worldwide (see metre, 1 E-2 m).