Does the length of a headline has to do with rankings and SEO? How?

The length of the headline should be short and sweet. It should contains the relevant keyword that is being targeted but do not jinx the headline in order to stuff keywords there. The search engines can only display a certain amount of title text and will cut out the rest making it look weird. Remember the best seo is often the best user experience - at least the white hat seo is.

the anchor text or clickable headline for results are important for adwords usability, SEO, and social sharing

i dont belived that length of headlines matters, head lines should be custom-tailored to suit audiences and optimized to meet different goals.

Yes. Lengthy headlines may not be eye catching for some people. It must be attractive enough to grab the attention otherwise lengthy headline is not the solution for getting audience on ad.

I think online readers have short attention spans and don't want to read long articles.

300 words may be the minimum ideal word count; however, research shows that posts containing more than 1,000 words rank best.