Normally the file contains some information which is the real world size it should be printed at but its often not correct and in any case not all file formats contain the same information.
If you want the tried and trusted way then look at the map and extract the perceived real world scale it should be. Thats easy when it has a scale bar or a 5ft grid or something like that but otherwise you will have to guess based on houses, carts, trees and whatever to give you an idea of the scale of the image.
Count it all up and get a real world width for the image. Get the pixel width of the image too. So say its 45ft wide image and a 1000 pixels.
Now get the scale of your miniatures which is usually 1:60 or 1inch = 5ft and that would mean that your map of 45ft needs to be printed 9 inches wide.
1000 pixels / 9 inches is 111 pixels per inch.
If your printer can be set in DPI or PPI then thats the value to use or else set it to print 9 inches wide. With a sheet of A4 or A3 you can usually tell by the print preview whether it will be about right.
We all seem to like PosteRazor here for chopping larger images up and turning them into multi page PDFs with the right sizes built in for direct printing. That might be another easy option.