Take care of this before cropping or resizing
Holding the left mouse button down, drag to select the area of the image that you want to keep (everything outside of the dotted line is what you want to delete). As you drag, a little pop-up box will display the width and height in pixels of the selected area.
When the selected box is the desired size, release the mouse button.
Canvas Size refers to the width and the height of the layer, or the working space, upon which your image is sitting. Enlarging the canvas size will enlarge the working space around the picture without enlarging the picture itself.
Decreasing the canvas size may cut away part of the image if the new canvas size is smaller then the image in height and/or width.