Reposition the two layers to fit the height and width of the canvas

Are you asking how to change the canvas size to make room for the reflection?

1. Image > Canvas Size.
2. Enable relative, click the top center anchor, then go back up to height and increase the size. Go bigger that you think—you can always crop later.

Relative means that you are adding the exact amount of space you need, i.e., 2 inches or 4 inches and the anchor at the top means the original image stays at the top, and the new canvas is added below.