Q: How do you print on letter size paper?

Are you using AirPrint?  Are you using an AirPrint compatible printer?  If so, there likely is no option to set paper size, but I would expect the printing operation to work from the Photos app.  So if the issue is with the printer, set the printer up to print on letter paper (using the printer's console, or the printer's paper tray depending on the printer's specifics).

Print from your iPhone, iPad, or iPod touch - Apple Support