Hi Jamie,
What I want to know is how the print sharpening in Q scales up
All you really need to know is that it is "smart" and once a level has been set in the options, it should work for all sizes of prints.
However, more detail can be read if you open the Edit - Preferences - Printing options. Click the Help button to see the details.
In simple terms what your do is sharpen your images so they look right to your taste on screen and then set the Smart Print Sharpening level so a print matches the screen level of sharpness. Start with the default of level of 5, make some test prints with different levels to get the one that matches the screen image. I think most users find the default level of 5 is fine.
You can set the level in the preferences or in Job Properties on the main screen, see screen shots attached.
You can read the on-line manual about this here
http://www.ddisoftware.com/qimage-u/help/function.htm#printinterp.
so will the halos scale up to work with the larger size?
If you can see halos on screen it sounds like the image is over-sharpened!
Terry