If you are trying to render in the orthographic perspective at a DPI (dots per inch) higher than 72dpi a few problems are presented that you may have already noticed. There are two workarounds to this problem. In a Camera Window- Set a camera so it has a 0 to 5 degree focal setting. This will give you orthographic perspective or something close to it. Then move the camera away from the object until you get the view you want and begin a new rendering. In a Modeling Window - Using the View Zoom Tool zoom in very, very close to the center of your desired image. Using one of the fast renderers (listed as RT fast, SL fast) run a test render. You will need to adjust the view with the View Zoom Tool as necessary.