Here is a post comparing the two watches on my wrist with pictures. This forum has been very helpful to me in the past (as a lurker) so I wanted to give something back to hopefully help future buyers choose between the two sizes..
I had admired the Stowa Flieger for a long time on instagram, but was unsure which size to buy. My most worn watch is the Longines Conquest Heritage 35mm and I usually didn't like watches larger than 38mm on my very flat and thin wrist. I bought the 36mm, but after three months of using it I decided I did not like it, and bought the 40mm.
In my opinion, the 36mm is not just a smaller version of the 40mm. It almost feels like a different watch:
- The bezel is slimmer on the 36
- The lugs are more prominent and more "straight" sort of. I noticed them a lot, in a bad way
- The case on the 36 feels more curvy
- The minute markers have the same width and the numerals are the same thickness on both size. This results in a more dense appearance on the 36. On the 40mm there is more air and space on the dial. All minute markers are lumed on the 36.
- The rivets on the 18mm strap looks tiny
I usually wear a 35mm watch, so I thought the 36mm would be perfect. But in my opinion it looked too small on me. There is something about the proportions between all elements on the dial and case on the 36 that did not sit right with me. It's small details I know, but small details are why we are on this forum? The 36 feels more like a dressier to me with more curvature and slimmer bezels, more delicate in a way. The 40 feels more like a tool watch.
My size recommendations (these are only my opinions of course). You should probably get the 40 if you have my sized wrist or larger. The 40mm is pushing the maximum size for me. My wrists are thin and very flat below the wrist-bone, so I can pull off a lug-to-lug of around 50mm with no overhang (if the watch sits low on the wrist). You should probably get the 36 if you have significantly smaller wrists than mine. Just keep in mind that they appear more different than you maybe had thought, not just in size. I had fallen in love with the appearance of the 40mm, and the 36 I bought initially was not the watch I fallen in love with.
In the end, it's just a few millimeters and not the end of the world. All watch collections have room for both small and large watches, and variety is the spice of life!
The seiko in the comparisons is the skx013.