Perhaps I didn't explain the problem very well. I know how to move the edge of the crop area, but I want to keep the same proportions or ratio of the image, so if I drag the top edge down I want a side edge to move in as well - so that the image will always be 6 "units" wide by 4 "units" high.
Understood,I'm not very familiar with PSP. Perhaps this is what you're after ; "! Click the 'Maintain aspect ratio' checkbox in the Crop tool Tool Options palette to ensure the aspect ratio is not changed when you move the sides of the crop rectangle. When you are happy with your selection, double-click to complete cropping the image!"