Tk409 took some wonderfully exacting measurements off a Mystery helmet that, when compared against laser caliper measurements of the ESB and ROTJ helmets on display have shown them to be close enough to the same size as makes no difference. He got a measurement around the base of the dome, above the brow band and without the rangefinder ear in the way, of 724mm. The baseball helmet I picked up is exactly the same.
I was even more impressed when, after removing the hideous earcaps it comes with, I took ~11mm off the back edge of the front half of the dome on my Hasbro Boba helmet and it now has the same shape as the batting helmet, and is 724mm around the base of the dome.
--Jonah