When to submit as a "Brief communication", when as an article? Many journals, especially in the life sciences, offer the possibility to submit results as as a brief communication.

I'm wondering when to choose this way of publishing. If I don't have enough results for a large paper because it used to be a "hobby project" that should be put to an end, but the results are interesting and I want to publish them, should I opt for a brief communication?

How do I choose between this form of publication and an article?

This includes the question of how small a "real" article can be.
The answer is often that you submit an article, and the journal says "We might accept this if you can make it a brief communication, which means 1 table, and X words".  (Where X is a relatively small number.

Write the article and see if it naturally fits into their criterion for brief communication. If it does, submit it as such, if not, submit it as an article. The length of a 'real' article depends on the journal, but I've had a lot of papers rejected and this only happened once because the article was too short.