Nevertheless, new researchers are well advised to submit short articles that make just one or two specific points and provide supporting evidence. It is much easier to get a short article published than a long one. And having more articles in total is an asset when promotion and tenure review comes along. One of the principal secrets of research is that no good research project is ever finished. There is no natural ending point.
Good research generates new perspectives and new questions and potentially goes on indefinitely.
You must arbitrarily cut off some part of the research, frame it for an audience, use it to make a point of interest to them, and publish it as is. You probably have already done X,Y, and Z but not included them. You will negotiate with the editor of the journal how much of this to include in your revised version, which will then most likely be published. That is how it usually works. The logic of a book is the opposite of that of a journal article.