The canvas is the visible area of the sprite. You can change the canvas size using Sprite > Canvas Size... menu.
Note: Resizing the canvas updates all slices accordingly, changing their position.
In this window you can change the Width or Height manually, or
using the blue rules in the editor window.
You can change the canvas size using the current selection bounds:
You can remove transparent borders automatically from the canvas using the Sprite > Trim menu.
