The math is the same as when scaling for web or email. You set the DPI of your scanner or the resolution setting of your digital camera to produce the right resolution for the size print you want to make. The accepted standard for digital photographic printing is to print at 300 dpi. Optical zoom enables you to zoom in on an object by using the lenses. Digital zoom enables zooming through the use of software.
Optical zoom provides a far better image than digital zoom.
This article pertains to the second kind. If you have a digital camera using AA batteries, it is recommended that you purchase nickel-metal hydride AA rechargeable batteries. You can find these NiMH batteries at most any store these days. Look for ones that are at least 1.2 volts and 1200 mAh. If you buy 1600 mAh batteries, they will last longer.