Hey OP,
There is no exact answer for your question.
You can write as long as you are ranking for your keywords are ranking. But if you are writing an article about some topics means please make sure it is not less than 300 words.
It has to be unique and should contain valuable information to the visitors.
Apart from that,
I couldn't understand why people are stick with only one option for their ranking.
Content quality, Other on page optimization and backlinks, Utilizing the social media power all these are important for ranking.
You can not expect the ranking on medium and high competition keyword by targeting any one.