You need to have your background selected, it is usually the 'image, if you opened it first' so that will be what is changed in size with canvas.
If you do not want to add extra on the outside of your scene you could scale the background up a little. You would need to select just the background layer to scale, use image > scale.
If that still doesn't work you could also scale your portrait layer down so it fits on your background. This time layer > scale.
If you do not want to add extra on the outside of your scene you could scale the background up a little. You would need to select just the background layer to scale, use image > scale.
If that still doesn't work you could also scale your portrait layer down so it fits on your background. This time layer > scale.
