<kbd>Ctrl-Alt-E</kbd> <kbd>Ctrl-Alt-W</kbd> (Windows) or <kbd>Ctrl ⌘ E</kbd> <kbd>Ctrl ⌘ W</kbd>(Mac) opens the "Edit guides" dialog. There you can set the exact position of the two guides. Using this and snap-to-guides and a little math, you can set the exact dimensions of images and shapes. Lots of monkey-motion, but it will do what you want.