"Change Canvas Size" command in Canvas tool lets you change the size of the canvas.
Open ① the Tool Selection windowand select ② Canvas tool.
Select ① Change Canvas Size.
① Keep Aspect Ratio, which will keep the given ratio between the width and the height of the canvas.
Turning off the aspect ratio will let you change the size of the canvas freely.
② Width(px) can be changed by pixel.
③ Height(px)can be changed by pixel.
④ Origin lets you align the canvas.
Tap on ⑤ ✓ to finish changing the canvas size.
Start drawing after changing the canvas size.
Finished.