Answer:
A summary sentence should brief the whole content “what so ever the length of the original content” may be.
For example, if you take a story, moral will be the good example of summary. One another example is when the teacher taught concept in the classroom, in the last few minutes of the class the teacher would brief the whole into smaller points.
Even nowadays, people go and visit movies only after seeing the review online. So once again the review is a small brief about the movie in one or two lines. It should be crisp, use cherry-picked words, etc.
