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.
2009 Waterford RS-14 S&S Couplers - Brooks B68-Anatomica - Traveller
2008 Waterford RS-33 - Brooks B68-Anatomica - Go Fast
2012 Waterford Commuter - Brooks B68-Anatomica - 3.5-Season/Commuter
2011 Surly Troll - Brooks B68 Imperial - Snow Beast