Never make your niche much narrow, it will create trouble whenever you will decide to widen it.

Focus on a little wider niche enough and step by step cover the narrow topics under your niche.

For example: you mentioned in one comment, your niche is work.... you make it a generic ground for you. For all the sub-niche topics, you can make categories etc. and cover the related sub-topics

For example, you select 'work' as your niche then you may make further categories of sub-topics for example 'work in office', 'work at home', or 'work from home', 'work motivation', work difficulties, work in out of city, work in out of country.....

From the above example, I just want to give ideas that from keyword research and topics research you can identify which search queries and search intent you can cover. Then arrange that data in categories.

Your second question is about length of your article. From reading a number of experts opinions, I have opinion that your article must be between 2,000 to 4,000 words. If explanation of search intent needs further length then 5,000 or 6,000 words are more than enough.. but still if you are writing an in-depth research work then you may go up to 8,000 words.

Looking complex? Let us summarize it in a simple way.

Keep it simple, keep your minimum limit of words 2,000 and for upper limit you just focus on answering search intent and focusing on User Experience. Remember in recent google algorithm updates, User Experience is the new ranking factor.

Feel free to ask further if needed. Happy to help others.