The term "sterling silver" is mainly used in relation to silver, and for such metal to be called as such, it must have a minimum degree of purity, which is what the law establishes in Spain or in most EU countries.
In this specific case, regarding the term 925 sterling silver, that 3-digit number signifies the quantity of precious metal in a piece. For example, if it's a 925 silver earring, it means that 92.5% is that precious metal, and the rest is another metal.
Earring measurements: 17mm long, 11mm wide, and the hole measures 3.5x1mm