Coming off hot summer want wanting some options for fall, decided to get a pair. i'm about 15" in the upper arm but automatically ordered a large based on measurements I must have taken wrong and some fits I had done in a store last year. Too small. Swapped for X-large. They barely fit and I guess that's the top of the sizing range, so I'd say order a touch bigger than the sizing would indicate. I tried them on other day as temps fell to 68-72 and I was feeling chilly. They helped and were mostly adequate for the slight drop in temps but not much more. They felt chilly in the wind and shade and I don't think I would want to wear them in much cooler weather.
For some reason unclear to me now I thought these were men's sized, but they're unisex, so being a guy that wear's medium men's size I needed to return that pair and get a large unisex size. I have always been happy with Pearl Izumi arm and knee warmers, and these look like they will be great once I get a good fitting pair.