This winter we had an unusual cold spell in Calgary and I had to bicycle about 18 miles round trip at a temperature of -33 C. I had a large fake fur cap with ear flaps that you tie up under your chin, and had just discovered that there was room for a toque under it. The 2 of them together made a really nice combination that kept my head warm and comfortable for the ride.
At these temperatures I'm not usually speeding along and in addition there was uncleared snow for this ride, so I wasn't too uncomfortable about not being able to fit a helmet over all that, but I have had an accident where I landed on my head and crushed a helmet, so it was just a bit of a worry.
I have gone into 3 large stores and checked every helmet they had for something a little larger that would do the job, but came up empty, but finally online shopping seems to have an answer. Here is the url for a $40 Bell helmet that fits from M to XXXL, (58 to 65 cm, 22.75 to 25.5 inches). http://www.bellhelmets.com/index.cfm...ategory_id=236
It is just a one size helmet that you can make adjustments to, to accommodate that range of sizes, so that would let a person vary the amount of keep warm stuff they have underneath. I haven't ordered it or tried it out yet so can't really make an enthusiastic recommendation, but if I don't find anything better or cheaper by the time the temperature starts to drop again, this looks really good to me and will probably be the one I get.