An Abstract is a part of an academic paper (including dissertation)which, even though placed at the top, is written only after completing the document. It is a condensed form of the vital information, its analysis, limitations, and recommendation presented in the dissertation.
Thesis is supposed to be written after the completion of thesis. Writing it at early stage might make you miss important details to include. Restrict the thesis abstract to two paragraphs. Write it in a concise manner that your reader should get a clear idea what should he expect in thesis. Information in your thesis abstract and thesis must match.