If you're wanting to resize the canvas specifically, it's Edit -> Resize the canvas. Choose inches as the dimensions and enter the values you want ( 8.5 and 11, be sure to uncheck the 'keep the original painting aspect' box ). Choose the DPI you want to print at ( 300 should be decent, see how it looks on your specific printer ).
For more details, I'd recommend the section 'Resizing and Rescaling your work' in the ArtRage manual which can be accessed via Help -> ArtRage Manual, as this gives a more detailed explanation of the features.
I hope that helps!