So, if you want to change the size of your image, you open the image. I will choose this one:
Then go to Image » Image Size or press Ctrl + Alt + i. A new window will appear.
Choose the width or the height you want for your picture. Also you can deactivate the Constrain proportions property and your image will transform to exactly the the width and height you want. I Chose for my image the height of 200px and the width was filled automatically.