I have tried to solve this problem by myself without any success. My printer is an Epson 3880 connected to my Mac Pro via ethernet. It's rated as a A2 printer (17" for you folks on the other side of the little pond). Everything should be running latest or next to latest driver/software: OSX 10.8.2, Lightroom 4.3, Epson drivers 9.04.
However, no matter what I do the printer seems unwilling to print anything larger than A4 when I use Lightroom. It prints on larger papers, but it only seems to print an image that's about the size of an A4 minus the margins. And it's not the complete picture, just a part of it. Small 102x152mm (4x6 in) papers are filled correctly with ink as they should be. Everything seems fine on Lightroom when I press the Print button. The Lightroom preview shows as it should be, picture covering the whole page, but fails to deliver. PDF's work just fine when saved from the print window.
Photshop CS 5.1 seems to be able to print normally. The same goes to OSX's own Preview program. Neither, however, have the same printing controls for sharpening and brightness/contrast as Lightroom 4.
My problem solving so far has gone as follows:
1) Reset mac printing system, delete all epson drivers from /Library/ and /Library/PPDs/, reinstall drivers and printer. I've tried using other paper sizes and going back to the desired size.
2) Reinstalled 10.8 on a totally clean system, loaded the lightroom library file and tried to print just as normal, no joy.
3) Restored the old system, loaded Lightroom with a totally new catalog, imported a few pictures for a test, no joy.
4) Printed from Lightroom 4 in my old PC on the same network. Worked like a charm. So much for the much advertised ease of using a Mac, eh? 
My brain has now exhausted the normal mehtods for figuring this out, so I'm reaching out to the experts out there who might help me with this.