The case diameter on a watch’s spec sheet tells you less than you think. What decides whether a watch sits on your wrist or hangs off it is the distance from the tip of the top lugs to the tip of the bottom lugs. This page draws the case, lugs and strap at true size and checks them against your wrist.
Lug-to-lug against the top of your wrist
Lay a ruler straight across the flat top of your wrist, where the watch sits; that width, not the circumference, is the limit. A watch whose lug-to-lug is wider than it overhangs the sides, and the strap kinks away from the wrist. Curved, downturned lugs help a longer case sit on a smaller wrist.
As a starting point, watch sellers suggest a 34–38 mm case for a 150–160 mm (5.9–6.3") wrist, 38–42 mm for 160–180 mm, 42–44 mm for 180–190 mm and 44–46 mm above that, with lug-to-lug up to about 46, 50, 52 and 54 mm. Thin cases and short lugs wear smaller than their diameter.
Measuring a watch on screen
Take the watch off and lay it face-up on the stage after calibrating. Set the case size until the circle matches the case without the crown, then set lug-to-lug until the lines meet the lug tips. For square and rectangular watches, use lug-to-lug alone: it is the length that matters on the wrist.
Lug width is the strap size
The gap between the lugs, where the spring bar sits, is the lug width, and straps are sold by it: 18, 20 and 22 mm are the most common. Lay the end of the old strap between the two red lines on the stage and pick the chip whose lines just touch its edges.
Odd sizes such as 19 and 21 mm exist on many vintage and dress watches. An even strap in an odd gap either rattles or has to be squeezed in, so match the exact size.
Thickness and shirt cuffs
A case under about 12 mm thick slides under most shirt cuffs. Thicker divers and chronographs sit higher and wear larger, so if you are between two sizes, the thinner watch will usually feel smaller on the wrist. A bracelet (metal strap) also adds weight, which makes a large case feel bigger than the same case on leather.