Hi Umba! Welcome to the forums.
In order to make sure that printing directly from ArtRage is the correct size, when starting your painting ( File -> New Painting ) make sure to specify the size you want in cm, mm or inches and also set the DPI for reasonable print resolution, say 300 DPI. This will ensure that your image is of the correct dimensions and contains enough detail to look good when printing.
Regarding the second question, ArtRage doesn't do that currently, but we will be looking at file saving again for the next version of ArtRage. Thanks for the feedback!