This isn’t really a Dorico question per se, but it’s related:
Not having an 11x17 printer myself, I was trying to get a pdf of a 32-stave score printed by a commercial print company. In Dorico 5, I had set the 11x17 margins to an inch all the way around, and it looks great. But when I convert to pdf (since the service I’m using can’t print native Dorico files), the Mac I’m on adds huge margins to the file (in Mac Preview) before it prints. Like another couple inches all the way around. I can’t find any margin settings in Preview.
You did use the Graphics Export function (upper RH corner of the Print Mode screen)? If you can avoid it, don’t use the ‘dumb’ OS-provided print-to-PDF routines from the print buttons in the lower left corner. They produce lower-quality PDFs.
One issue (in my experience) is the addition of unnecessary extra margins, to “helpfully” fit your page on whatever size your stupid home printer is able to print.
The Graphics export respects your settings. It produces PDFs you can reliably print at 100%.