What’s your favorite case size?

I have an 8.375" wrist and I like 43 mm, though my two most recent purchases have been 40 and 41 and the smaller size is growing on me.

I have a 7 and a half inch wrist and anywhere from a 40mm to a 45mm is fine for me. I do have an old school tag heuer formula one chronograph ca1212-1,  and that's 38mm and it wears quite well. But nothing smaller would really work for me. I think most people who like dive watches would say 40mm to 43mm is the sweet spot. 

for small wrist i prefer:

non bezel watch (dresswatch, GADA) 36-40mm

bezel watch (gmt, dive) 40-43mm

I prefer 38-42mm usually. But of course other factors can allow for larger or smaller cases to fit well also.

40mm 

This is a good question. I am by no means a seasoned pro at this, but for my wrist, I agree with Unholy, as far as sizing for non-bezel and bezel watches. The thickness and lug width make a big difference to me as far as how a watch "wears" on my wrist, as I find the interaction with my shirts to be an important factor. From my perspective, case size is part of an overall series of factors that determines how a watch feels and looks on my wrist.

34 to 38mm.  Today's is a 29mm.  It reads well.

34-38mm

39mm is perfect for me 

I like this break down. I never thought about it based upon the functionality of the watch.

I found that I gravitate to 42 to 45mm watches although I love the look and feel of 36 to 40mm watches. I have a 7.5 inch wrist and for my over all size, 42 to 45 doesn't look ridiculous. At least I don't think.

As already said, it depends more on the lug to lug distance, and the shape of the case than the diameter of the watch might suggest. Also the type of watch. A diver or pilot's watch will tend to be bigger than a dress, or field watch. This said I have a 7,25 inch wrist and gravitate towards 36mm field and dress watches and 40-41 divers but have also a 32mm and a 43mm in the collection.

43-46mm lug to lug.

Diameter and depth to be proportionate the lug to lug numbers.  If those proportions are ok (to my own taste) then my OCD is suitably satisfied! 

I can appreciate any proportionate case size (even if I cannot wear it myself)

39mm, but none of my watches are exactly that. Closest is my O&W P-101 at 39.56mm (now there's some Zurich OCD). Range is 34 to 43. 6.75" wrist.

My wrist is 6.25" and I typically enjoy 36-39mm. However, it really depends on the ability of the watch manufacturer to make their case wearable. For example, the Seiko MM200 fits well on my wrist though it is outside of my "comfort zone" for case diameter. I really think it comes down to how well the watch is balanced, the lug to lug distance and the curvature of the lugs. If those specs are in line with my preferences, I'm willing to wear up to a 42mm. 

39-40mm

39-40. 

7” wrist here.  For me, divers from 40-44mm.  ”Normal” watches without a big bezel tend to look bigger, so 38-41 seems about right to me.

For my 6.2 inch tiny wrist,  38 mm to 40mm. Lug to lug less than 48 mm

wrist is 7.25". I like 40mm, 48 lug to lug, and 20mm lugs. The golden ratio for me

I'd say 40 or 41mm is perfect.  I have just over a 7 inch wrist.  My first "nice watch" was a Tag Heuer Formula 1 Chrono that I believe is 42 or 43mm... it felt nice at the time, but now I have a Tudor GMT and a Rolex GMT Master II and they are 41 and 40mm, respectively. My father has a 36mm date just and I think I could get one of those some day... I just like that most of my collection is around 40mm size right now.

Nice mixing it up with GMT’s. Also I don’t know what it is about the 36 Explorer and the Datejust, but in my opinion they look best in that case size, but I also know not everyone can pull those sizes off.

I've got a 7 inch wrist and my preferences read as follows:
Dress watch: 34-37mm

GAGA/dressy-sporty: 36-38mm

Sports watch with dominant bezel: 37-39mm

all within the confines of sub-47mm lug-to-lug.

I find the key measurement for my enjoyment of a watch is actually the case high. 13mm or below and most sizes a fine. 

I love my casioak which is 47mm case but only 12mm thick. 

I think 39 is the best spot for a medium size wrist, like my old IWC 3706. but a lot depends on the case shape…

All down to the lug to lug for me, I’ve had small diameter  watches which overhang due to the flat case back and a long lug to lug, and large diameter watches which hug the wrist. I often wonder how some designs ever get through the feasibility studies and prototype testing, it drives me mad as I‘m OCD about a perfect fit on my wrist. A lack of micro adjustment on bracelets should be a criminal offence in my book. 

Smallest one is  a 33mm vintage nivada, the biggest one is a 45mm seiko turtle. I don't wear them for the same occasions for sure but both serve a purpose.

By the way the dial on the turtle is the same size as my sinn 556 which is a 38mm. and 47mm lug to lug is shorter than most 40 mm watches. Which will be a sweet spot.

38mm and 40mm white dials vintage-ich dresswatches look definitely wrong to me next to my vintage watches. But a modern looking dress/tool watch up to forty is ok for me. And 37 38 mm skindiver / vintage style dive watch still look proportiniate. 

36-42 mm. But I’d go smaller or larger if I liked the watch and how it looks. My eyes can’t sense the difference in a mm or two.

I think it depends on the watch.  If it’s a diver with a bezel, 38-43mm; if it's a dress watch 38 and under depending on the size of the bezel. If it's a field watch that also depends on the design. And then there's the mid case, is it tall? Is it chunky? 
 

The best thing that I've done is try them on. It's like an article of clothing. They fit differently and one day you're in the mood for a heavy oversized dive watch on a bracelet weighing over 200 grams and some days you want the 35mm vintage reissue with acrylic crystal that disappears on wrist. 
 

This Tag Heuer Aquaracer is in my collection (43mm) and the Longines was in my collection (35mm, I think?) i like how they both wear. My wrist size is just a touch under 7"

Lug to lug is important and so is the bracelet. A watch can be heavy but the bracelet may mitigate the weight. I think there’s such an art to designing these things. That’s why you don’t see the microbrand cases from the 2000’s with their slab sides en vogue anymore. Buyers have moved on. 

35-36mm 👍