When you change the print size of an image and you're going to a different aspect ratio, make sure the values you enter will accommodate the whole picture, otherwise your print will have gaps along two sides. Of course, the trade off is that a small part of your image will be clipped.
Take, for example, a typical digital camera file that you want to print at 4 by 6 inches. With aspect ratio restricted, you can either do 6 by 4.5, resulting in a half inch of the height being clipped. Or you could do 4 by 5.333, resulting in a blank strip along the top and bottom edge of the print.
To avoid having the image arbitrarily clipped by your software or photo printing service, you should crop the photo to the correct aspect ratio, prior to setting the print size. Many newer software will have preset aspect ratios in the crop tool for common photo print sizes. In Photoshop and Photoshop Elements, you can enter the height and width in the options bar before making a crop selection to crop to a specific aspect ratio--just avoid putting a number in the resolution field if you don't want the image resampled when you crop it.