The format of an annotated bibliography can vary. It's important to ask for specific guidelines.
The bibliographic information: Generally, though, the bibliographic information of the source (the title, author, publisher, date, etc.) is written in APA Style.
The annotations: The annotations for each source are written in paragraph form. The lengths of the annotations can vary significantly from a couple of sentences to a couple of pages. The length will depend on the purpose. If you're just writing summaries of your sources, the annotations may not be very long. However, if you are writing an extensive analysis of each source, you'll need more space.