An abstract is only included in a long piece of writing such as project or dissertation. It is the academic word for a summary. For a 15,000 word dissertation it may be one or two pages in length. The length of the abstract is defined for some dissertation writers but not others. It is best to look at samples of other dissertations from the same course if possible - ask your supervisor/lecturer, check in the Perry Library.
Acknowledgements are included in projects and dissertations and are not usually longer than a third to a half a page. Authors briefly mention the people who have helped them complete their writing e.g. academic staff, other students, family, friends.