The canvas is the white area in the center of the system (graphics outside this area will not be displayed after saving). Cropping the canvas allows to quickly adjust its size to fit the graphic being drawn.
Click the Canvas Crop icon 
Drag over the portion of the canvas you wish to retain to create a selection box. The selection box does not need to be precise, as you can adjust it later.
If necessary, adjust the crop selection box:
To move the selection box, place the pointer inside the box and drag it.
To resize the selection box, drag a handle.
To complete the crop, press Enter. To cancel the cropping operation, click outside the selection box.
