Resolution depends entirely on the type of output device. If your output is to be done on press, then you need resolution equal or greater to half the line screen (lpi). Most commercial printing is done at 133 lpi, some at 150 lpi. For that type of output, youi'll need 266ppi and 300 ppi respectively. (although, I've used as little as 200 ppi @ 133 lpi with acceptable results.) An inkjet only requires resolution of 150 ppi. If your render is small enough, you may be able to render normally, so that you can continue instead of rendering to disk.