All canvas resize commands accept math operations. So, if you want to double the canvas width (i.e: when you are creating a CSS sprite), you just hit Ctrl + Alt + W and add ‘*2′ to the width in the prompt.
Canvas Height (CTRL + ALT + H)
Sets the height of the canvas.
Canvas Size (CTRL + ALT + S)
Sets the size of the canvas.
Canvas Width (CTRL + ALT + W)
Sets the width of the canvas.