OK, I bit the bullet and bought a pair of the sale bar mitts. Took them out on a balmy winter day (about 32F/0C). I probably could have ridden with just summer cycle gloves. Toasty warm.

I just bought another pair for DH and some for the winter bikes (flat bars). Ugly as can be, but they really do the trick.

Size-wise, I went with medium. I had this fear of getting my hands in and out quickly. I could probably have gone with a small except that I have FSA Wing road bars (flat tops). This makes the bar mitt take on a funny twist, effectively making the opening a little smaller.

That said, the medium is about 17" around at the opening; once on the bike, you have about a 12" loop of bar mitt above your handle bar.