The Demo version of ArtRage Studio Pro is limited to 1280 x 1024 pixel size canvases. It also is limited in export to JPG files, with a watermark.
20" x 8" at 600dpi is probably going to be too large for ArtRage to deal with reasonably in any case. That's 12,000 x 4800 pixels, and ArtRage needs more memory per layer than Photoshop because of the additional paint properties per layer.
If you're making an image 20" in size, consider if you really need 600dpi. For viewing an image that size the user is unlikely to be close enough to distinguish between 300 dpi and 600 dpi in any case. Even 150 dpi viewed at a distance of 36 inches can be adequate.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"