What you need to do is talk to a print company who can produce the final print and ask them what size the image you are printing needs to be. When they tell you the size, start your painting in ArtRage by selecting the Print Size option in the New File dialog and enter the dimensions and the 'DPI' the print company asked for. This will ensure that the image is the correct size for printing. When you go to send the image to the print company you will need to ask what image formats they can use (ArtRage Paintings are not a standard image format) then use the Export option in the File Menu to save a copy of your painting in a format they understand. Usually, print companies will ask for a format like TIFF or PSD, both of which can be exported from ArtRage.
Hope that helps!