Something I always try to warn my customers about is enlarging and cropping.  It seems I'm never able to explain this very well, so I found a "visual aide" that demonstrates this. Notice how when you go larger than 4x6, some of the photo will need to come off on the "short ends".  This is because the photos I take are formatted for 4x6, so the proportions do not stay the same as you get larger.  See below.