Here you select the new width of the image. Since Keep aspect ratio" is checked the height will follow automatically.
Here you select the new height of the image. Since Keep aspect ratio" is checked the width will follow automatically.
Keep aspect ratio
By default this alternative is checked so that the image will scale proportionally. If by some reason prefer the image disproportional, uncheck this alternative.
There are different types of scaling (i.e. how the image editor adds/removes pixels):
Nearest neighbor (fastest)
For a fast but least precise result. This method might result in an image that feels a bit "edgy".
Gives medium quality.
Bi-cubic (best quality)
Slowest method, but it will render the most precise image with soft colors. When you edit a jpg image you will get a new jpg image, but if you edit a gif image a new png image is created since gif is a license protected image type.