They are a stretchy -it'd be worth trying one on. I used mine all through Storm Dennis when walking the dog (2hrs at a time), my head stayed perfectly dry and I didn't need to use a hood - perfect!
I have considered these hats before, I liked the idea of a waterproof and windproof beanie.
Never for wearing under the helmet, I have various scullcaps and a headband for that, more for when I'm waiting at the train station, those platforms get proper cold and windy, especially if a bit damp or wet from the ride, or when locking up the bike and wandering about.
Part of the warm down process, or the warm up if I get to work proper frozen.
Is it a big one size, or is there a fair bit of stretch? Large head.
I have a nice Salomon one at the moment, always lives in the bag. But not wind or waterproof. Fits nice though.