Each print size has its own ratio - you can do the math easily by dividing the long side of the print into the short side. My example was a 5 X 7 (7/5=1.4), but let's consider an 8 X 10. In this case the aspect ratio is 1.25 (10/8=1.25).
So, to adjust the image size in my example to that aspect ratio, I would multiply the short side by 1.25 (2048X1.25) to get required pixels for the long size of 2560. Subtracting this amount from the existing 3072 pixels on the long side gives me a crop of 512 pixels to be taken off the long end in order to end up with a 1.25 aspect ratio for an 8 X 10 print.
Did not realize there was so much math in photography, huh? The easy way to do this, of course, is to use the crop tool in Photoshop or Elements where you simply enter the required print size and dpi and position the crop tool on the photo as you would like it cropped. However, it is good to understand the math involved. It is also something to keep in mind when composing photos in the camera, as you will likely lose some of the long end when you print.
Out of curiosity, what camera and editing software are you using?