Here are my helmet measurements. Only one problem - I haven't the faintest idea if they're accurate or not!
The helmet I'm (slowly) building is 220mm wide at the bottom of the dome and 250mm from front to back of the dome.
The width between the sides flares out to 260mm at the base - but that's minus any of the ear pieces. The front and back will be parallel. If I am lucky it will fit my head without the need for a nose job.
The black vertical visor will be 55mm wide and the depth of the top of the 'T' will be 25mm where it meets the vertical bit, and narrow down to about 5mm at the ends.
As for the height - can't remember without going out to the shed, but the whole point of this is that measurements are really only part of the story.
The reason that Natty's helmet looks so stunning is that he has got the various ratios spot on - you know, the angle of the side flares, and the shape of the dome, the width compared to the height, etc. We've all got different sized heads and a big range of heights. So what's right for one person may be way off for another.
Like I say, the measurements for what I'm building are based totally on looking at photos, making numerous cardboard mockups and printing out pictures and measuring the widths to get an idea of the ratios. I am on the fifth prototype now. Fortunately I don't have to please anybody except myself and my son I'm doing this for as a present.
People might be reluctant to share the exact dimensions of gear they've spent ages finding the genuine dimensions of, and I can certainly appreciate that they would treasure these measurements. I think that out of respect for Natty's awesome work and his generosity in posting photos from angles that let you work out the ratios, all I could recommend is to study his pictures and work out something that's going to be right for you, but use some of my measurements as a starting point if it helps.
Cheers!