Hello, welcome to the forums!
When you start ArtRage, by default your painting will be at screen resolution ( 72 dots per inch or DPI ). Printers will tend to use a higher resolution so when you print out, your image will tend to be smaller on paper if you haven't specified a physical size.
However, you can set up a painting in the following way when you begin:
1/ Go to File -> New Painting
2/ Select the size in inches or cm of the result you want to see when printed
3/ Select the DPI setting. 300 DPI should give you decent quality.
Then when you print out, your painting should be the dimensions you selected.
You can also change the dimensions of an existing painting by going to Edit -> Rescale the painting and setting the size and DPI values there. The downside to this is that if your painting is quite small and low resolution to begin with, enlarging it will reduce the definition or overall quality when printing at the new larger size.
( moving this to the Tech Support forum )