Can't print multiple PDF files in 11x17 format.

Adobe Reader can be real pain it the... neck sometimes.
But first I would inspect not only the printer setup, but also printer's default print settings. Windows XP has 3 places, where you can see the printer options:
1. Printer properties
2. Print preferences
3. Default print setup.
The third one is the one you have to set. When you double click on your printer icon, a print queue window opens. Choose Printer->Properties, go to Advanced tab, and on the bottom there's a button "Default print setup" (I don't have English XP handy, so I'm translating back to English from my version - excuse me if they might appear a little different).
It's there you need to set up the default page size and so on.

If this still doesn't help, try opening Acrobat Reader, setting the Page Size Scaling to None, printing something and closing Acrobat Reader. From what I've seen, it tends to remember this setting from previous sessions.

You may also try alternative PDF readers, like Foxit Reader or PDF Xchange Reader.

Good luck :)