How can I set 4 x 6 as the default printer setting? I'm working on a photo booth web app that's going to run at a kiosk, and I'm trying to figure out how users can make a 4 x 6 printout of their photo.

When the user requests a printout, PHP resizes the photo and moves it into a folder being watched by Automator. When Automator sees a new photo is runs a "Print Images" action, but the action doesn't have an option to change the paper size, so it prints at the default US Letter. It seems this could easily be accomplished by setting the default paper size to 4 x 6 in the Print &amp; Scan options, but that doesn't seem to be a choice:  

Is there any way to add custom options to this list? Of course another way to accomplish this would be fine, too.
If you print from the command line with lp you can do something like:

lp -o media=Custom.4x6in filename

That will print at 6x4 on the default printer the file called filename.

On my HP inkjet that made it pick the photo tray over the main tray.

So, put the above in a script and have automator run it pointing at the directory with the files in.