You would use image size if you wanted to reduce the size of the image, but say you cut a render from a wallpaper and your left with a massive canvas. If that's the case go image-canvas size and change it there.
if your using ps and you just cut out your render and pasted it on a new layer .. go to Edit > trasform > scale . keep the shift key pressed when resizing and every thing will stay in proportion..
if ur using ps what i do is click help->resiza image.... lol... seriously....its gives you a preview before you click ok so you can make sure its the right size