Yes, you can adjust the canvas at any point. Any new strokes will reflect the flat canvas, so you don't want to switch back and forth while you're working unless you want that effect.

PNG is usually the best file type for general printing, but if you aren't going to be editing the exported file further then a high quality JPEG will be fine and might let you scrape in under a certain number of MBs (go to Edit > ArtRage Preferences > Advanced Preferences to adjust JPEG quality).