Diameter sets the category, not the fit
Case diameter is normally measured across the case without the crown. It helps distinguish a compact dress watch from a substantial diver, but bezel width and dial opening change the visual result. A 40mm diver with a broad bezel may look smaller than a 38mm watch with an edge-to-edge dial.
Use diameter to narrow a shortlist. Do not use it to buy before trying the watch or checking the remaining dimensions.
Lug-to-lug controls the footprint
Lug-to-lug is the distance from the upper lug tips to the lower tips. If those tips extend beyond the flat top of the wrist, the watch can look and feel unstable. Curved lugs may wrap more successfully than straight lugs with the same published length.
Fixed or male bracelet end links can extend the effective length beyond the official case measurement. Examine side photographs and handle the bracelet rather than relying on one number.
Thickness changes comfort and proportion
Thickness determines cuff clearance and how far the centre of mass sits above the wrist. A thick watch can remain comfortable if the caseback settles into the wrist and the strap holds it securely; a flat slab-sided case can feel larger than its measurement.
Compare thickness within type. A 12mm diver may be impressively slim, while a 12mm formal watch may feel bulky.
Measure the wrist, then test the whole watch
Measure wrist circumference where the watch sits, but also notice its flat width and shape. Two wrists with the same circumference can present different surfaces. Begin around 34–38mm for a 14–16cm wrist, 36–41mm for 16–18cm and 39–44mm above 18cm, then deliberately break those starting points when the design calls for it.
Check crown clearance, clasp position, bracelet taper and whether you can insert a fingertip beneath the strap. The correct fit is secure without leaving persistent pressure marks.