This tutorial demonstrates how to change the size of an image.
You can change the size of your image at any time, by going to the "image" tab and then to "image size" (or with the Alt+Ctrl+I keys).
Here is the table that will appear:
1 – The height and width of the current image (before the change)
2 – Type here the height and width you prefer for your image
3 – The new resolution
4 – If you check this choice, the height and width will change synchronously, and if
you uncheck the second choice, the resolution and the volume will change (if the resolution increases, the volume decreases).
See this video: