The Boushh helmet that I made I scaled it to fit my wife. The over all dimensions of the helmet size I used are 11 inches high, 12.375 inches wide & 14 inches deep. This in my opinion is larger than the actual. I would concur with your 13 inch (shoulder to shoulder) cause Carrie Fisher was rather petite & she was only 5'1" tall. I believe the actual helmet size is around 11 inches wide. From what I can tell by scaling reference pictures.
Someone else may have better information. Hope this helps some.
