The sizes available for your print depend on which media you will be printing on − some medias require you to select a pre-determined size, (as with certain types of posterboards). Others allow you to choose any custom size you wish.
If the media you have selected allows you to choose any custom size, you may enter one dimension, (such as the desired width). The other dimension will be automatically calculated based on the image you are resizing. This allows you to enlarge the picture without stretching it or distorting the aspect ratio.