Use Edit -> Liquid Resize for content aware resizing. This effect uses a technique known as seam carving which attempts to resize the image while preserving its contents. This is useful for changing the aspect ratio of an image without stretching or cropping important areas.
Use the width and height fields to change the size of the image in pixels.

Note that some images may not be suitable for liquid resizing, in which case the Resize layer or Crop layer would be more appropriate.