Thanks for the repsonse, JD.
I haven't yet been set up on any other printer before this, so, no, I haven't printed anything yet on 11x17 anywhere.
I have been in the printer properties, and for this one there is a tab for "Paper/Quality" where I can set "11x17". However, the next tab is called "Effects" and has a resizing section where it automatically picks "Letter" each time, and there is no "11x17" option in the list. There is also no option for disabling this resizing, so I'm not sure how to change it. However, even if I change teh resizing to A4 or something, it still comes out on Letter.
Again, like I said, I know this is proving to be an HP issue, but I was just hoping for someone with previous experience or "know-how" to help out while I try and get a hold of HP support.