My specs: I'm 5'4", ~110 lb., typically an XS/0-2 in women's clothes, with slim/average arms for my size.  I ride my road bike 3-5 times per week on trips of ~30 miles or less.  I got these arm warmers in cobalt (blue) in XS, and they fit great -- right on the money in terms of sizing.  The color is true to the picture, like a royal blue.

It has started getting cold here at dusk/night, and I get cold pretty easily to begin with.  I've brought these arm warmers with me on three rides now and was so thankful I had them.  Fortunately, they fold up very small and take up almost no room, so I was able to squeeze them into my seat bag.  They have a grippy silicone elastic band at the top to hold the warmer on your arm, and the inside is lined with soft fleece.  Each time I've worn them so far, I've been wearing a thin, short-sleeved jersey.  I put the arm warmers on when it got down to about 60 degrees, and they kept me comfortable the whole time -- I didn't even need to put on a jacket.  (When we stopped for water, I actually got a little overheated and had to pull the arm warmers down!  They are SERIOUSLY WARM!)  I can anticipate using these arm warmers a lot, especially if there will be a wide temperature variation, if I don't want to bring a bulky jacket, etc.

The only downside to these arm warmers is that they're a little short on me, and I'm not tall by any means.  I didn't mind the small gap (maybe 1 cm) between the top of the arm warmer and my jersey sleeve, as I found that actually helped regulate my temperature.  (As a disclaimer, I don't wear traditional road biking jerseys but instead slim-fitting, wicking athletic tops, typically with cap sleeves, from Athleta, Lulu, etc.  These arm warmers would probably be the perfect length, with no gap, for traditional road biking jerseys.)

Overall, highly recommended.