ArtRage currently doesn't have a means of adjusting the dpi of a painting - we always assume a screen resolution of 72dpi.
If you need a specific size and dpi, first caclulate how many pixels you need: pixels = dpi x inches.
Then when you've painted your image and need it printed at that exact size, export it as a png or psd and take it into another application such as Photoshop where you can adjust the dpi to suit.
AndyRage's mantra for graphics engine code:
"Sure - how hard can it be?"