This is my first pair of Castelli cycling gloves (I'm just beginning to get into Castelli gear now). Took them out twice for rides that started in the low 20F (11F with windchill) and ended in the high 20s, maybe hitting 30F. Started out a bit cold, but once I got warmed up, the gloves kept me nice and warm for 30-40 mile rides. Would think they could go below 20 and still keep my hands warm.
My hands measure 7"from base of palm to top of middle finger. That makes me either a small or a medium per Castelli sizing. BC only had the medium so that's what I got. It fit well, a little bit long in the fingers, but not excessively so. It was still easy to change gears, brake, etc. I have tried on the Lightness glove and the small fits great, but the small Chiro Due might be a bit too tight. If they had a small Estremo, I would try those.
The gloves are fairly thick relative to other cycling gloves I have (Gore Xenon), but you feel that as extra warmth when you start out. The palm of the glove feels a little 'fleshy', like there was too much material there, but it didn't interfere with handling the bike at all.
Great glove, bought a second pair as soon as BC had another one in. Highly recommended!