Do you use GiMP as well? Make a frame or backdrop of flat grey (or preferred for your palette) of the desired pixel sizes. Then make sure you ONLY paint on layers on top of that. Using a "frame", you may have that layer hovering above your work instead. It doesn't need to be more than a border of 1 px width, and in a 300 dpi painting, it'll be virtually invisible. That is not necessarily a GOOD thing. You need to know where your boundaries are when painting as well

Open the .ORA in GiMP, select your frame or canvas layer and autocrop (
Image -> Autocrop image) the image. This function is available in most major commercial image handlers as well. When saving a flat .TIF or .PSD more suitable for print, make sure to remember pushing the resolution up
