When to submit as a "Brief communication", when as an article?
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.