How do I change just one dimension in Preview, when dragging/using the mouse?
in Preview with the picture open:
when it fills the window completely, click on it with CMD - (command plus the minus sign).
this will give you manipulation space.
then place the mouse pointer inside the picture, a + sign appears, drag this to where you want to start, a blue drag line appears, place it around the picture completely; then when you want to the picture smaller drag that side, and click Tools->Crop in the menubar of Preview.