Lot of people don't quite realize this, but watch sizes should be proportional, and here's a nice illustration from Gentlemen's Gazette,
Ideal Dial and Lug Sizes
The Slender Wrist's excellent article, take this further with a reference table.
What I've done here is more a summary for myself and my table is a little different because my Excel formulas does not round,
Watch Case Size
Lug To Lug Distance
|Flat of Wrist*|
|5.25 "||13.335 cm||26.67 - 33.3375 mm||26.67 - 42.2275 mm||44.45 mm|
|5.50 ”||13.97 cm||27.9 - 34.925 mm||34.925 - 44.23833333 mm||46.56666667 mm|
|5.75 ”||14.605 cm||29.21 - 36.5125 mm||36.5125 - 46.24916667 mm||48.68333333 mm|
|6.00 ”||15.2 cm||30.48 - 38.1 mm||38.1 - 48.26 mm||50.8 mm|
|6.25 ”||15.9 cm||31.75 - 39.6875 mm||39.6875 - 20.27083333 mm||52.91666667 mm|
|6.50"||16.51 cm||33.02 - 41.275 mm||41.275 - 52.28166667||55.03333333|
|6.75"||17.145||34.29 - 42.8652 mm||42.8625 - 54.2925||57.15|
|7.00"||17.78||35.56 - 44.0375 mm||44.45 - 56.30333333||59.26666667|
|7.25"||18.415||36.83 - 46.0375 mm||46.0375 - 58.31416667||61.38333333|
|7.5"||19.05||38.1 - 47.625 mm||47.625 - 60.325||63.5|
The formula estimates the flat of your wrist. In some you may actually have a smaller or larger wrist than the formula and should adjust accordingly.
We use the preferred ratio case diameter of 60% to 75% of the size of the flat of wrist. Rough estimate of flat of wrist is your wrist size (in mm) and divide it by 3.
Ideal lug to lug distance is 75% to 95% of wrist width.
Again the Slender Wrist article, "The Ultimate Watch Size Guide: Factors Influencing The Size Of A Watch" has a comprehensive discussion and these are my short notes,
Colour - white looks bigger and black looks smaller) and lug to lug distance also plays an important role here,
Bezels and Hour Marker Circle - The watch design also visually impacts the eye to see a watch smaller than it is,