Change your font type to something that takes up more room like Palatino Linotype or Calibri. Additionally, the larger font often looks more professional because it's cleaner and easier to read.
Highlight the whole text (Ctrl + a in most word editing programs) and increase your font size by .5. You can usually get away with a whole font size larger but you might not have to, depending on how much longer you need to make your report. Remember, the more changes you make to formatting, the more likely they are to be noticed.
Change the paragraph settings in your paper so that the breaks between each line and paragraph is longer. You can do this easily by highlighting all of your text and clicking Paragraph. Go into Spacing and increasing the line spacing and the "After" spacing. A good rule of thumb is to increase it by 25% or more; if you originally had 1.5 line spacing, increase it to 1.75, and the paragraph breaks from 12 pt to 15 pt.
Change the margins of your paper very slightly by highlighting all of your text and moving the margin lines by 2 or 3 ticks on the ruler at the top of the page (beneath the toolbar). If you don't see the ruler, make them visible by clicking View>Show/Hide and check the Ruler box. You can alternatively highlight all your text, click on Page Layout>Page Setup>Margins>Custom Margins and increase them by .2 using the arrows.
Change the font size of your headings and paragraph titles by highlighting the heading text and using the arrow keys next to the font size to increase the font by 2 points. This will make your headings stand out and add to the length of your report.