I just rendered to disk an image for a poster at 600 dpi 24x36. Although you actually only need 300 dpi for print I wanted to make it even crisper since it will be so large. The result was great, as was said Bryce does the math for you and although it took me 2 days to render to disk that particular image size, the outcome was well worth it. It's always better to render at the required size or larger because let's face it you can always scale down but you can't scale up.