Changing the canvas size of an image changes its overall size without changing the size of any of its contents. In short, the editable canvas of your image increases or decreases, but whatever was on the canvas doesn’t change. Increasing the canvas size adds additional space around your image, whereas decreasing the canvas size essentially crops the image. However, the cropped areas are not deleted, they are simply hidden beyond the edges of the canvas.
Enter a new canvas size for the image or use text field math to calculate it.
Select the Relative checkbox if you’d like to specify how much to add to or subtract from the current canvas instead of entering a specific canvas size.
Move the canvas box to choose where the original image should be placed on the new canvas.
Click OK to apply your changes.