I do ALL my images high resolution for print at 8.75" X 11.25" (to account for bleed --- The trim size is 8.5" X 11"). What I do is set my Carrara render output to match the Photoshop pixel size/count I am after. For example an 8.75" X 11.25" image at 300 dpi is 2635 pixels by 3375 pixel (inside Photoshop). In Carrara I set the render output pixel size to 2635 pixels by 3375 pixels at 72 dpi. That was 72 dpi NOT 300 dpi (if you input 300dpi you will be sorry). Select Keep Proportions and your image should turn out to be about 33.8 mb. Not that bad for a single image. I then render the image, usually as a tif. When the render is complete I take the image into Photoshop. The image size is 36.458 inches x 46.875 inches at 72 dpi inside Photoshop. I Then change the size to what I am after. All I do is constrain the proportions and set it to 8.75" X 11,25" at 300 dpi and I have high resolution image high enough for high end printing. It ususally does not even take very long to render.