Click the Size drop-down menu [1]. You can select a small, medium, large, or extra large image size [2], or you can select to enter custom dimensions [3].
To customize the size of your image, enter the custom height or width dimension in one of the two dimension fields [4]. As you enter a value in one field, the other field's value changes in order to retain the image aspect ratio.
Note: Custom image size must be at least 10x10px.