Lot of people don't quite realize this, but watch sizes should - in general - be proportional, and here's a nice illustration from Gentlemen's Gazette,
The Slender Wrist's excellent article, take this further with a reference table.
My table is a little different because from the Slender Wrist's as my Excel formulas does not round and I've also added an ideal strap sizes column,
Watch Case Size
Lug To Lug Distance
|Flat of Wrist*||Strap Size (common)|
|5.25 "||13.335 cm||26.67 - 33.3375 mm||26.67 - 42.2275 mm||44.45 mm||65/110 or 70/105|
|5.50 ”||13.97 cm||27.9 - 34.925 mm||34.925 - 44.23833333 mm||46.56666667 mm||65/110 or 70/105|
|5.75 ”||14.605 cm||29.21 - 36.5125 mm||36.5125 - 46.24916667 mm||48.68333333 mm||70/110 (my own preferred custom size)|
70/115 (works well too)
|6.00 ”||15.2 cm||30.48 - 38.1 mm||38.1 - 48.26 mm||50.8 mm||70/115|
|6.25 ”||15.9 cm||31.75 - 39.6875 mm||39.6875 - 20.27083333 mm||52.91666667 mm||70/115|
|6.50"||16.51 cm||33.02 - 41.275 mm||41.275 - 52.28166667||55.03333333||70/115|
|6.75"||17.145||34.29 - 42.8652 mm||42.8625 - 54.2925||57.15||75/120|
|7.00"||17.78||35.56 - 44.0375 mm||44.45 - 56.30333333||59.26666667||80/125|
|7.25"||18.415||36.83 - 46.0375 mm||46.0375 - 58.31416667||61.38333333||80/125|
|7.5"||19.05||38.1 - 47.625 mm||47.625 - 60.325||63.5||80/125|
The formula estimates the flat of your wrist. In some cases, you may actually have a smaller or larger "flat of the wrist" then use ratio.
The preferred ratio case diameter of 60% to 75% of the size of the flat of wrist. Rough estimate of average 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.
Women may consider (if going more feminine) more tapered straps and bracelets.
I prefer quick release straps (to put reference and picture below) so you can change your straps without tools.
Also, consider most bracelets wear bigger than straps.
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,
As with any rules, they are meant to be broken and a matter of personal preference.
I suggest considering your body type, dress, personal style and even facial feature will impact watch size.
You might have small wrists, but have broad shoulders due to swimming or lifting weights. In that case, you can pull off a bigger watch. But if you dress with slim fitting clothes, you might want to adjust closer to your wrists mid-range.
Finally, it's all about your comfort level and personal brand so rules are meant to be broken. However, I hope this article opens up some thoughts to not be afraid to try out different sizes and experiment.
The Slender Wrist - "The Ultimate Watch Size Guide: Factors Influencing The Size Of A Watch"
Martu - Chart for Custom Watch Sizes