If you need to crop a selected part of the image you will need to:

Note: A dimension indicator is located in the bottom right of the screen. This shows the width and height of the crop area and is a useful tool if you are preparing images to fit a specific size. The image below shows the crop area and the dimension indicator.

The following window will appear.

Check the actual size of your newly cropped image to ensure that the physical dimensions are within the recommended limits outlined in Image size. To do this:
The Resize window will appear, showing the pixel dimension of the image. If it is outside the recommended limits, you will need to resize the image.