That depends on the printer/settings you're using and what quality you need. How fine a printer can print is determined by the density of actual physical dots it can achieve and measured in DPI (dots per inch). A very typical value would be 300 DPI. When you do any printing in Xtreme Photo Designer 6, it will give you a screen where you can basically freely scale the photo on the sheet, though. This way, you won't have to do any calculations how large your image will turn out. You can just adjust it in relation to the sheet size.