Hi JEB,
I'm printing to an HP 8750 on an iMac running OS 10.7.2, and PixInsight 64 bit 1.7, and it works fine.
So, a few questions:
1) What model HP printer, and have you installed the latest Mac OS Printer updates?
2) What version of PI, and 32 or 64 bit?
3) Can you pull up the Printer Dialog using the Printer Setup button on the PI Print page, and if so can you set the paper size there?
Yours,
Sean