I checked the helmet reviews and settled on the Petzl Elios, size 2. Great helmet, except it doesn't fit my noggin. Couldn't even buckle the chin strap and I had the adjustable wheel all the way open. What helmet is equal to the Elios and will fit a huge noggin?
I'm no onomastician, but find it odd that my name is Gear Guy and people send me questions about climbing gear, while your name is Moorehead and your head is too big to fit a climbing helmet. Then again, we do still honor the 13th century Christian tradition of basing family names on occupation (Cooper, Mason, etc.) and personal characteristics (Whitehead, Dicknose, etc.), so maybe it isn't so weird after all.
American heads have, due to better nutrition and electromagnetic fields from cell phones, been getting larger for the past 50 years, but your gourd is off the chart. You say you couldn't fit the Elios, size 2! That helmet expands to a maximum circumference of 63 centimeters, typical for a size large and a reasonable maximum considering American heads are 53 to 61 centimeters around, with 57 centimeters being the average.
I will assume that you don't have an Afro and that you have removed the packing materials from inside the helmet, and go from there. I checked all helmets, and it looks like you are at the bottom of the birdcage. There is only one helmet that might fit you. It is the Grivel Salamander XL. This ABS-shelled, polystyrene-lined helmet is similar to the Elios, but it lets out all the way to 66 centimeters, large enough even for Gorgons.
But, I wonder if your problem isn't so much a large head, as it is a large jaw. You said you can't buckle the chin strap. Perhaps you share DNA with Jay Leno. I don't know. I do know that if the difficulty is caused by your jaw, you might solve the issue by simply finding a helmet with an extra-long chin strap. Strap lengths vary from model to model. Get your pumpkin down to a gear shop and try on every helmet in the house, and remember: never eat anything larger than your head. Next!