You need to place your mouse-cursor near the edge of the crop-guide outline, if you move it around a little you will see an arrow. Then grab the very edge of the crop-guide and stretch or shrink. If you put the mouse-cursor near the corner then you will be able to stretch horizontally or vertically but without moving the base of the crop-guide. Helps to keep a careful eye when you do this.
If you attempt to move whilst the mouse-cursor is nearer to the middle then the entire crop area will shift. ie: your mouse-cursor needs to be at the very edge.