To make your size adjustments for height, drag the corner box up and down. Make sure that the cursor is a little diagonal double-ended arrow before you start dragging. If it is, then you'll resize the picture. If, instead, the cursor is a little cross of double-ended arrows, then you'll move the picture.

As you drag the corner of the picture, an outline of the picture border will follow the mouse, keeping the correct width-by-height ratio (or aspect ration).