I have been working with GIMP a lot lately and also playing with MyPaint. I have seen some very good paintings done with MyPaint and I love the feel of the brushes. But I have always been able to control my canvas size and resolution. But MyPaint doesn't allow you to set either from scratch. This is one method that gets around those issues.
I created a template in GIMP and set the dimensions to what I need. I left the resolution to 72dpi due to the fact that MyPaint will only save to 72dpi (at this time.) I can get a good print quality as long as the canvas size is larger than I need. For example, an image with 2400x1332 resolution at 72dpi has a canvas size of 33 x 18.5. Setting the resolution higher than what you plan to use it for offers the benefits of cleaning up brush strokes. The last two things I do is add a visible border around the edges on a separate layer. I use it as a visual reference when I import it into MyPaint. The last thing is to save it to the OpenRaster format (.ora).
Hope this helps,