Sterling silver — 92.5% silver, alloyed for hardness — is measured by weight, so a chain that costs little is either very thin or not sterling. At 1.2 mm this one is deliberately substantial enough to survive daily wear rather than being the hair-thin chain common at lower prices, which tends to kink and then snap.
The clasp is a standard lobster, which is the type most people can work without help. If you have difficulty with fine clasps, say so when ordering and we will substitute a larger magnetic closure at no charge.
Lengths are measured end to end including the clasp, which is the standard convention. If you are buying to a specific neckline, measure a chain you already own rather than guessing.