Corel PhotoPaint and Draw are designed just for this type of tasks. But like Jason said, you need to know the resolution (dpi) of the output device (printer).
A 900x300 pixel bitmap image would print out as a 9" x 3" image on a 100 dpi printer, a 3" x 1" image on a 300dpi printer or a 1.5" x 0.5" image on a 600dpi printer. I hope that this gives you an idea how that work.
That is just a basic example. Many programs also let you print using adjustable scaling where you can print the bitmap at some percentage (like 50% or 200%). Then the printout size gets resized by the specified percentage beforem being printed. Simple, isn't it?
The bottom line is that more advanced bitmap editing or publishing programs (like Gimp, Inkscape, Corel or Adobe Illustrator) make this easy as they allow you to specify all the dimensions so you know exactly what the printout will look like,.