While the crop image wizard can be used to change the shape of the image, you can also use the RESIZE option to reduce the dimensions.
This can be handly when you have uploaded the file direct from your phone as-is meaning the filesize is excessively large.
You can view the original image size dimensions below the image.
Click the blue dropdown arrow to choose a preset size, or set your own max dimensions via the Width (Max) and Height (Max) fields.
With the Crop to exact resized dimensions left unchecked, the image will be resized within these values, while maintaining the current aspect ratio.