Wish to paint dog in scenic background. What size should dog be?
I don't know any formal artistic composition rules but how I would handle it is by mocking it up with either Adobe Photoshop or Gimp Photo Editing Software.  (Gimp is free).  Put the background in one layer and the dog in another, then scale and move each until it looks good to you.


Then, just for fun... I apply an oil or watercolor filter to see what the finished product will look like.


Then I hire someone with talent because without a computer to do all the hard parts...  I suck at painting!