Ctrl-alt-e crtl-alt-w (both key sets in that sequence) 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.

Ctrl-Alt-E Ctrl-Alt-W (Windows) or Ctrl ⌘ E Ctrl ⌘ W(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.