Resize an Image:

Open up the image you want to resize. (File > Open)

Go to the Image menu at the top > Scale Image.

A Scale Image window will come up. Change either the width or the height to what you want it to be. It will adjust on it's own. For example if you type in 200 for the width, it will change the height automatically so the image is still proportionate.

When you've changed the width or the height, click Scale.

If you need any help - ask me here.