Alternative UI Discussion

With our decision to use buttons extending from the corners, we have come to a dilemma as how the canvas size should be handled.

In the example UI above, we have our corner buttons in black, with the sample canvas size in grey. In a possible full canvas, you can draw behind the buttons, which is disconcerting to the user and hides a portion of the drawing. In a possible small canvas, you could shrink the canvas size, but would minimize the impact of the corner buttons, since they would not be in the corners... We are unsure how to proceed and are leaving any options open to discussion.