When you crop it is changing the image size, it wouldn't make sense to change them independently since you would basically be undoing what you had just done.
You can select aspect ration in the crop tool to whatever you want and then no matter the size of your image, it'll always "fit" to the page without clipping any of your image off, as Mark said, when you fit to page on the print out (as long as your printer prints all the way to the edge of the page, otherwise you need to add bleed which is a whole 'nother story).