I think starting out either way is fine honestly, but it's important that a new creator who's leaping into comics and having to make this choice understands the consequences of either path, both positive and negative.
I'm a big proponent of the "Start with some smaller works first before leaping into something large (Magnum Opus or not)" approach which I hadn't really thought of as an approach even until I started frequenting these forums in 2017. I approached the topic as someone who started on the other side- I started out by working on this big mega huge shonen anime-inspired story that I came up with in ~2007 (7th grade) and continued to work on here and there until... 2016ish? Mind you not much of that was actual comic drawing, it was mostly just drawing and refining the characters for years, re-doing the first 5 pages of chapter 1 like 10 times, and thinking/writing out story ideas for the future.
The main pitfalls that I see with starting with a longer series (and experienced most if not all of these myself) are:
- Not having a good idea of how long these things take and never finishing
I naively thought that since a lot of manga series released chapters on a weekly or even monthly basis that I could do that to, and thus I wrote my story out to be super duper long without concern for how long any of it would take. Even at my current drawing speed that story would take me an insane amount of time to produce. But since I had never taken another crack at drawing something longer than like 5 pages prior I didn't know that I couldn't draw that fast.
The main cause of a lot of people's re-draw loops (mine included). Especially while you're still early in your art career and developing rapidly it's not uncommon to quickly outgrow your earlier art- especially while drawing a comic which naturally speeds up the rate of grown through variety and quantity. First impressions aren't everything, but they do matter, so it's natural for people to feel uneasy when the first chapter or 2 of their super duper long story is kinda sub par compared to what they can do now and worry if that's turning people away from the project as a whole.
The benefit to starting with a shorter project or two or three is that you don't have to worry about the art at the beginning of the story too much because it won't be that long before you're past the whole thing and onto the next one... and when you start a new story you have a new chance to leave a kick ass first impression with all of the skills you developed last time~ Now of course even after your art stabilizes a little bit it will always continue to grow if you keep pushing yourself as you go... but the key is simply reaching a point where the art that you start a longer story with is satisfactory enough to not make you worry. The point where you can say "Well Chapter 10 will look way better than Chapter 1, but chapter 1 looks good enough!"
The same as above, but with the technical parts of putting a comic together- paneling, word balloons, fonts, etc. I think people tend not to cringe over this quite as much as their drawings, but it's still something to be mindful of.
Something I've heard a lot on this topic is that it's important to practice writing and drawing not only beginnings and middles, but also endings. Just like there's nuance and techniques to set up the call to action and continue to propel the momentum forward throughout the middle section of a story, there are also different ways to finish it out effectively, and this is something you just may never experience if you start a 20 year long story lol.
- Feeling of accomplishment
This is highly subjective and will certainly not be the case for everyone (well that goes for the whole list tbh) but a lot of people (myself included) get a rush from crossing the finish line and having a complete story under their belts. There's just something rad about being able to finish something and it can provide a good platform from which to launch forward into an even stronger start. I even had finish-ception going on with my most recent project-
I had been working on a 70 page short story last year, but last spring I was kinda starting to burn out (around the 2/3 point maybe). An opportunity came up to draw a short 10 page comic for an anthology so I put the main project aside for a minute and cranked that mini-comic out in a couple of weeks. The rush coming off of finishing that gave me the motivation and drive to finish the 70-page comic strong afterwards~
So like... I mean your mileage may certainly vary. I especially think that if comics are just something you're pursuing for fun as a hobby, that just working on whatever sounds the most appealing is the way to go regardless of size.
If you're looking to take it farther (whether to try and make a career out of comics, or just taking the hobby more seriously to build following or whatever) then starting small to hone your skills and working your way up is a better approach. Theoretically any comic can blow up, but honing your skills to make a project as strong as it can be makes the process less luck-based (or at least helps to turn the odds in your favor).