print.always_print_silent - anyone used this option before?

I'm using the "print.always_print_silent" option with Mozilla 1.4 under
Linux to suppress the print dialog box and print directly with the default
options.  I've added the preference to unix.js and the diaglog box is
suppressed.  However, now I get a message that says:

"There was a problem printing because the paper size you specified is not
supported by your printer."

I've played around with what appears to be the paper size setting,
"print.print_paper_size", which was originally set to 0, but I've also tried
3 and "letter."  I've also tried the same with the
"print.postscript.paper_size" setting and I get no change.

The printer I'm using is an HP 3650 with CUPS and Foomatic.  I don't have
any trouble printing to it with the dialog box.  Anyone have any ideas?

Thanks
Matt