when you hear Castelli you default to size up. Thats not the case with the nano arm warmer. The medium conforms nicely although I do have thinner arms... true to fit. The material is thick without the heaviness. There is the elastic band material to keep it from slipping.
I use Nanoflex on damp days or when there is rain in the forecast. I think they have a little less stretch to them than the Thermoflex, but they are warmer and provide some nice protection from the wet. 6'-1160 and medium fits me well. They do stay in place nicely despite a little less flex. i may pull them up once or twice on a long rides if i make a stop, whereas the Thermoflex don't budge on me. Or i can just ride through and not worry about them. seem well made, only been through a few washes but no signs of any wear.
These do the nanoflex thing really well and are sized reasonably well. I'm an extra-medium kinda guy, and the mediums fit pretty well. Like the nanoflex 2 leg warmers, these are a little long, but that's okay. It took me a bit of time to decide which was left vs. right and how the anatomical cut is supposed to go over the elbow-just remember that when you're on TV people should be able to read nanoflex or castelli.