It is generally standard for an annotation to be 200-250 words (one paragraph). Below is a real 7-source annotated bibliography. Check it out to see how the format and sentence structure work.
This is an example of one full-length annotation. Notice how, by the time the author reaches the end, they have answered all 9 of the Engeldinger questions.
Also notice how the answers to the questions are implicit--the student has written sentences that weave the answers together as a coherent annotation and not as a patched-together list of information.
In MLA style.
Vogel Library, Wartburg College | 100 Wartburg Blvd, Waverly, IA, 50677 | Phone: 319-352-8500 | Email: firstname.lastname@example.org