Q: How can I change the size of a photo when printing from iPad?

What app are you printing from and what printer are you using? From the Photo app for many printers you just need to put the right size paper in to the printer. Here's a discussion about the HP printers with photo trays:

Using AirPrint to Print Various Photo Sizes

For more basic printers you can try an app like pita4print which will let you pick a paper size.