I was born in Chicago (Cook County Hospital) so I thought I'd add my 2¢.
Before you spend any time, or work in iPhoto, check the settings you used on the camera to record the images.
Many cameras offer different settings but only one is worth sending out for prints.
The key is DPI (dots per inch) and they are stored inside your image file.
If you captured in a low size setting (72 or 100 DPI) your multi mega pixel camera may only allow a "print" size of 5X7 (maybe even smaller).
Next setting is "medium" and it uses 180 DPI. Again, if you crop the image, you print size will be on the small size.
Last is "large" and it is usually 300 DPI. This is what a printer wants to use as it gives the best results and brings you back for more business.
Today you can drag your images (Finder to CD) and bring them into a Wal-Mart for developing.
So. If your images are large (300DPI) you'll get good results when printed. Smaller captures means smaller prints.