Image size can be specified with menu Image/Image size. You can either select a predefined image size or type your own. If you type your own, you have to type in a format "widthxheight", e.g. 123x88, or percentage, e.g. 35%, or "width", e.g. 1024. The former is absolute size and the latter is relative. When you type your own, you have to press ENTER key to accept the change.
See also:
Image menu