If anyone tells you that it’s “quality over quantity” and that a short one’s fine, they’re lying. Quality is essential, but the fact is that a quality short one isn’t going to do the job as well as a quality long one.
That being said, quality and skill does matter – don’t for a moment be fooled into thinking that you can get away just by having length alone. Each sentence you write should either be a (new) explanation, an example or an evaluative comment. If it’s none of these, you’re wasting your time writing something that won’t add content to your piece.