Martin, you can always adjust your selection by choosing Select -> Transform Selection. If you want to maintain the same aspect ratio, be sure to hold the Shift key while dragging any of the corner handles (not the side handles or you'll lose the aspect ratio). Hold Shift and Alt key together while dragging any of the corner handles and your selection will transform from the center point (all corners will expand or tighten simultaneously).

We do not see things as they are. ǝɹɐ ǝʍ sɐ sƃuıɥʇ ǝǝs ǝʍ