Hello. According to the link below, most competitions limit your abstract to 250 words, but check with your teacher and science fair to be sure.http://www.sciencebuddies.org/science-fair-projects/project_abstract.shtml
The abstract should relay the most important information from your experiment in a condensed version of your report. Pretend you are an outsider interested in your experiment and conclusions. What information is important for you to learn without having to read the entire report? The link above gives great advice on what to and not to include in your abstract.
I hope this helps. Be sure to write back if you have further questions. Good luck!