If you don't have any other program go to the top of your screen so the menubar appears. Click on the "files" menu and pull down to the "document setup" heading. A window shows up with the page dimensions. Change to a smaller dimension and render the scene. Save the resulting image under a different name than your final render. I usually add tm for thumb after the original name like "space.jpg" for the original and "spacetm.jpg" for the thumb. It makes the thumbs easier to find if you have a lot of files in the directory. Either resize again and save the file or quit and don't save your changes to keep the original rendersize.