You've chosen a journal that looks like it is well suited to your background and topics.
By itself, the length of your paper is not related to whether it is important or publishable.
There are three things you can do to test whether your potential topics/papers are worth writing and submitting. First, read many articles in this journal, similar journals, and also conferences. You aren't just looking for articles on the same topic or technique as yours. Instead, you are learning about what constitutes a good research paper in your field. Based on your reading, write a detailed outline of your proposed paper(s), and also an abstract.
The second thing you can do is to write a version of your paper(s) as a "Working Paper" and then circulate them to peers or colleagues for feedback. You can post this to Arxiv and post links on social media or mailing lists to get broader feedback. This approach has two advantages. You sometimes only know the value of a research paper after you finish writing it. And you'll get the most useful feedback from people when they have a finished paper in their hands, as opposed to reacting to a concept.
The third thing you can do, building on the previous, is to submit a paper or two (each revised based on earlier feedback) to a journal or conference, and then learn from reviewer comments if they reject you.