It should be no longer than 200 words, but usually it will be shorter. In published reports, i.e. journal papers, it appears in small print immediately after the title and it usually takes the form of a single paragraph. In unpublished undergraduate project reports it should be presented on a page of its own after the title page. You must ensure that the abstract can be fully understood without reference to any other part of your write up. The abstract is one of the hardest things to write in a report.
Most people do it last although it appears first.
An indication of what was investigated.