I don't see any harm in sharing the dimensions. This particular helmet is no longer in production. As long as no one is pulling molds directly from one of these, everything should be cool. The main goals of this forum are to identify and locate "found parts" on the real suits and to help each other make the most accurate Fett costumes we can. Sharing info is what it's all about.
Good luck with your helmet projects and be sure and post some progress pics.
::Edit:: It is really O.K. to blame Ego. 
