You can resize the document by changing the height/width in the Document palette and pressing Document>Resize. You can save this as the starting document size by pressing Document>Save As Startup Doc. Pressing New Document when the WSize switch is on will give you a new document that takes up all available space.
A document can be larger than the screen size. You can use the Document>Zoom to show all. You can also hide the interface buttons by clicking TAB, closing the shelves by clicking on the dividers and hiding the top menus by pressing the Menus button at the top of the UI.
Only by having a document size that has sufficient border around it. The border is for navigating when zoomed in on a model.
Yes, I tried the TAB key, but it doesn’t change the canvas size, though it does remove shelves. I also tried the zoom function, but it just enlarges the pixels, not the canvas as such. So I assume that the largest possible canvas is the one you can get by starting a New Document with WSize on. I guessed I hoped you could do what you can do in Word or Blender and use the entire screen as your work area.