It’s probably not bad advice to start with a short comic if you’re unsure about what you’re doing. You can learn a lot about technique, experiment with a style you like, and so on.
I started my first comic with this in mind, thinking it might be a project I would do for about a year and then reach the end. I ended up having so much fun with it that it’s turned into a near decade long thing, and still going for the foreseeable future. Weirdly, I’ve done more short comics since starting my long, main project, since it gives me more room to experiment and wander around with ideas.
Is my comic perfect? No. There’s a lot that I’m going back to fix and change in preparation for doing a book, so taking the time to plan, test, and experiment earlier on could have saved me that hassle. But on the other hand, I really love digging into a longer story this way and I’m not sure I would have kept going if I were limiting myself to a shorter story.
So I guess it depends on the way your motivation works, in my opinion.