I'm sorry, in the current version of Draw it's not possible to change the canvas size once you've begun drawing. I know it doesn't do you a lot of good for this drawing, but that is a feature that will be in the next version of the app (which should be available in a couple of weeks).
You'd have to send to the desktop and then resize.
I hope that helps a bit.
It's not yet possible in Draw to create custom canvas sizes; for now, it would probably be best, when beginning a drawing, to choose a size that's "close" (perhaps a bit larger) and then size down when you send the work to the desktop.
The drawing teams are working on custom canvas sizes.
Do we have a guesstimate on when resizing the canvas will be an option? This would make my life a much happier place. I have so many drawings that are on the wrong size canvas.
I've been told that custom canvas sizes will be in an update this summer; can't be more specific than that.
Hope that helps.