Why size is not just diameter
Case diameter is the number most often quoted, and the number most often misleading. A 40mm watch and a 40mm watch can wear entirely differently, because diameter is only one of three dimensions that matter. The others — lug-to-lug and thickness — often determine whether a watch sits well on the wrist more than the diameter does.
A watch that wears well is a watch you wear. A watch that is too large or too small tends to sit in a drawer, and a watch in a drawer is a poor value regardless of its other merits.
Lug-to-lug: the measurement that matters most
Lug-to-lug is the distance from the tip of one set of lugs to the other, measured along the wrist. It is the dimension that determines whether a watch overhangs the flat top of your wrist or sits within it. A watch that overhangs catches on cuffs, looks unbalanced, and never quite feels right.
As a rough guide, a lug-to-lug under 48mm fits most wrists; 50mm is the upper limit for an average wrist; beyond that you need a larger wrist or a different watch. Two watches with the same 40mm diameter can have lug-to-lug of 44mm or 52mm, and the difference is immediately visible on the wrist.
Thickness and how it reads
Thickness affects two things: how a watch looks and how it feels. A thick watch reads as sporty and tool-like; a thin watch reads as dressy and refined. A thick watch also catches on shirt cuffs and sits heavier on the wrist; a thin one slides under a cuff and disappears.
There is no correct thickness, but there is a correct thickness for how you wear. A 14mm dive watch is appropriate for its purpose; a 14mm dress watch is not. The proportion of thickness to diameter matters too — a wide, thin watch can read as elegant; a narrow, thick one can read as a hockey puck.
Measuring your wrist
Measure your wrist with a flexible tape at the point where a watch sits — just below the wrist bone. Under 6.5 inches is a smaller wrist; 6.5 to 7.5 is average; over 7.5 is larger. These are not rules but starting points. A 38mm watch can look right on a large wrist if its proportions are balanced; a 42mm watch can overwhelm a small one.
The most reliable test is to try the watch on. Measurements predict, but the wrist decides. If you cannot try before buying, buy from a seller with a return policy and be prepared to send it back.
The case for smaller
Watch sizes grew steadily from the 1990s through the 2010s, driven by fashion and the visibility of large watches on larger wrists. The trend has reversed. Smaller watches — 36mm to 40mm — are returning because they suit more wrists, slide under cuffs, and read as considered rather than loud.
A smaller watch is not a lesser watch. It is often the more difficult design, because proportion is harder at a smaller scale. The watches that have lasted — the references collectors still chase — are mostly modest in size. That is not coincidence.