Q: Printing to full size paper from iPad photos

The Photos app when printing to inexpensive printers always tries to print to the photo tray. When printing to better printers that know what paper sizes are loaded then the Photos app will give you a choice of the loaded paper sizes.

What printer model are you using?

Photosmart eStation C510? That's a 2010 model printer. From the Photos app it will only print to the photo tray. But the HP ePrint app, available from the store, will likley let you print from the main paper tray.