I would like to read shorter series and finish them, but I mostly read long series and lose interest as we get past chapter 200.

The hell I know. Currently there are more ongoings than finished series. The good ones will continue for 10-20-50 years.
Also, there are manga chapters from 1 page to ~200 pages length. Webtoons even can't be calculated.
Not representative.

I picked "short". Actually, most are around 60-90 chapters but there's a strange gap in the poll from "less than 50" to "over 100". I've read several longer ones as well but often I start losing the earlier plot midway through because they're just too long. Or I lose interest because they release too slowly. Some of the best series have been short-ish, even less than 40 chapters.

I suppose I do read 19 Days, but, as said above, the chapters are really short. As for Detective Conan. Most of it is episodic and you don't need to remember what happened. Developments in the underlying plot are rare enough that you won't forget. My earliest series were actually fairly long ones... and I don't remember any more than the bare bones of plot LOL. So shorter is better. I think a really good plot doesn't need several hundred chapters. Most of that will be filler or more unnecessary arcs to keep the money coming. It gets boring.

I don´t care how long a series is as long as it stays good but I have to choose medium by default here as not that many manga go above 20 volumes if we factor in market realities. Monster is 18 volumes for example and a lot of ground is covered. 5 options might have been better to give more range.

Hajime no Ippo is the longest runner on my reading list, its length can´t be justified, with JoJo thereafter. Its length is easily justifiable due to the character, time period and setting shifts. One Piece rounds out the Monster Trio and it too is overlong despite the problematic use of off-screening. The pre-time-skip era was better written and had reasonable pacing. (In the manga.) https://en.wikipedia.org/wiki/List_of_manga_series_by_volume_count

Mostly 100-200 chapters? I love to read oneshots or shorter series too (less than 100). Sometimes I also read manga/manhwa with long chapters, such as Bleach or Tomb Raider King. But I honestly can't stand a long running series with thousads of chapters.. especially if it's still ongoing. The main reason why I haven't read One Piece until now. 😂 Even I stopped reading Tomb Raider King after around 200 chapters and only pick it up again after it ended.

They are mostly one to two volumes. Up to 4 volumes is my favorite length! I love slice of life, so it does not require a really long story. You get to know the characters, and have time to develop the plot in that length.

Medium for me... Even though i don't really have a preference and read all the lengths... Super long ones tend to be few and far between for me because need to find time for them, and while i do enjoy shorter ones and do read some one shots, around a 100 chapters seems to ge what i have been reading most often lately...

I feel like we're all bound to read more shorts than superlongs, since the number of shorts will end up outnumbering everything after a while. Like- for every long one we continue reading, there's sooo many short ones that come out and you can instantly consume in the meantime.

What happened to 51-99 chapters?

100-200 is sometimes too long for me. Less than 50 is too short.
I find 70 chapters is a good length to keep my interest.

Picked medium as around 100 chapters is what I read a lot, but if there was a 51-99 option, that would have been my pick as well. I have read some superlong series but it's only a handful in the end, prolly have read more short ones but I guess they are just not as memorable to me often (with some exceptions).

I read so many manga, and lately a lot of them are actually pretty short and it's piling up mmm...

I'd say I prefer short-medium (50-150 chapter) manga. I started with shorter manga that have less than 50 chapters. But now I like longer manga.

There comes a point where the number of chapters puts me off. Somewhere around 200 chapters or above is where I'll at best read the first couple of chapters to check whether it's worth the investment, and around 400 chapters is where I'll never even bother checking unless it seems to be exactly what I'm looking for, and even then I'd be reluctant. I prefer something between 50 and 100 chapters.

I mostly read josei, shoujo, and seinen romances, so I am wary when they're above 50 chapters. 50-100 is more the range, but to be conservative on this, I picked short (less than 50 chapters).