The reason we limit the size on the iPad is due to the iPad's lower memory when compared to your desktop system. Unfortunately if you were able to exceed these limits it would negatively affect the stability of the app as the iPad would be more likely to close the application to free up resources.
The grumpy Mona Lisa may also indicate a corrupt file but it sounds like this isn't the case here.
If you have ArtRage Studio or Studio Pro, you could use script recording on your iPad when you create a new painting, then export the script and play it back on your desktop version to repaint the image at a larger size.
Resident Bug-Hunter and Support Guy