I've seen peer-reviewed articles of over 24,000 words, and short monographs (books) of barely over 100 pages. Is there a demarcation on how long can an article be without becoming a monograph? I'm in Philosophy.
Journals and conferences may have length limits on articles but they differ widely. A given publisher with many journals may have different limits for different journals. These limits are usually easy to find. Reviewers also make some suggestions about length on occasion if they find extraneous (or missing) materials in a paper.
Limits might be in words or pages and there might be both lower and upper limits in some cases. Journals that publish paper versions might be quite strict with limits.
The same publishers of those journals might also publish monographs, though some publishers specialize in one or the other. Each will have its own standards about what it publishes. Many journals consider monetization of what they publish and length may play a role in decisions, though it is unlikely to be a major consideration with other things such as timeliness and relevance (and market) being more important.
But articles and monographs are somewhat different in tone, with articles usually leaving much of the background of an idea in the references and monographs often including more of that for completeness.
If you have an idea for an article or a monograph, you can discuss the idea with an editor (or two) who will give you advice on which might be appropriate and also about how complete and self contained your work needs to be to be considered a monograph. If they find your ideas interesting enough they will work with you on publishing them. The length is something that will be a consequence of that.