The way I see it, is that even if it's a long comic that you're doing as your first comic, it will be short anyway. Kind of like how as a teenager I wrote out my long epic fantasy onto MS Word, spending years revisiting the document in my spare time, deleting stuff, rewriting stuff, really putting my heart into it. Came back to it my Sr year of college after I forgot about it for 6 or 7 years and it was only 28 pages.
As for comics, my first comic was an epic long comic, but it only updated for a year before I ended it early and did something different. I've done that to about 4 (5?) different stories, before doing a short comic that I did finish. (I have only truly finished one comic and it was because it was part of the webtoon contest and had to be finished to count) and then started on the comic I'm doing now.
Only now do I feel confident that I have the discipline and skill to finish a comic, as well as enough knowledge about the industry to help more than like 5 people see it (because despite how many comics I've made, I was basically working in a vacuum, no one cared.) And I don't really worry that my other works were unfinished (especially since one was fanfic so like eh I can't monetize that one anyway) because I can always finish them later.
I think the idea that you're doing a short comic first is because you are assuming that you'll have a ton of readers that you don't want to disappoint or something--which no you're not. Your first comic will rarely have like any readers at all. So like--just do the story you really want to do for as long as you want to do it. If it feels to stressful to take on that long of story then yes, don't do that story, but if there's only one thing you want to draw right now--do that thing.
Like I spent a few years more than I should have before starting that first comic, and it looked hella dated when I finally made it because it was really meant for a different time of my life when that look and that storyline was more trendy. I'm glad I finally started it, becuase it helped me make other things, but I think I would have been happier if I had made that earlier when I was a goof that could barely write 28 pages of a MS Word document. It would have been a 4 page comic, don't get me wrong, but I would have really enjoyed it.
But like as someone who's in her 30's--trends come and go so freakin fast. When I was a kid every comic was steampunk: now steampunk is hella dead. When I was a kid everyone was doing a tim burton alice and wonderland thing: now that's dated. Hell when I was as a kid, so many comics were using flash to try and be Homestuck, really interactive works that were almost video games--now that is an impossible format. We have trends now that I don't need to name, but we can basically guarantee no one will care about them 3-4 years from now. So get it out of your head while it's still fun.