If you ran the installer and don't have any other sizes then you did not link it to the printer, because the driver he mentioned works 100%.
That's already been installed. It doesn't help. Anything I try to print only shows 8.5x11. I try to update the media size and the dropdown that usually shows all the different media sizes shows only 8.5x11 and "Defined by Driver" as options.
You need to go the the www.efi.com website and download the new driver directly from there. The file name is: XC_WorkCentre75xx_v1_0R_FD51_v1Cert.dmg The latest driver versions for the Sierra OS have "Cert" at the end of the name.
I'm having no luck printing on 11x17. In fact I can't print on anything other than 8.5x11. It seems that the problem occured once I updated my OS to Sierra. I checked and you don't have a new fiery drive for Sierra yet. Any tips on printing to different sizes in the interim?