Portrait Lighting Setup poster to be helpful. It contains 24 different portrait lighting setups using a variety of lighting gear and lighting patterns. Click the image to see it up large. Also check out some of the further reading.
Resolution - Print Size Chart The numbers on the chart stand for dots per inch (dpi) they say the human eye does not see much beyond 96 dpi, that should give you a good example of what will work to make a good quality print.
How to Photograph The Night Sky - Getting Away From Light Pollution