My specs: I'm 5'4", typically an XS/0-2 in women's clothes, with average-size/-length legs. I ride my road bike 3-5 times per week on trips of ~30 miles or less. I got these Sugoi leg warmers in black in XS, and they fit great -- right on the money in terms of sizing.
It has started getting cold here at dusk/night, and I get cold pretty easily to begin with. I've worn these leg warmers with me on three rides now with temps down to 50 degrees and was comfortable the entire ride. They have a grippy silicone elastic band at the top to hold the warmer on your thigh, and the inside is lined with soft fleece. There's a zipper on the back of each calf with a reflective covering. The XS is a perfect match for the length of my legs, given where my cycling shorts hit on my thigh.
I also have a pair of Nashbar leg warmers (size Small), and while those get the job done, all I can say is: you get what you pay for. If you need leg warmers and you're on a budget, by all means, go for the Nashbar ones. But if you can afford to pay just a little more, get the Sugois; they blow the Nashbar warmers out of the water because:
1) They're warmer. I feel far less wind through my Sugois, and the fleecy lining is cozier and more thermal than the Nashbars.
2) The silicone band holds up the Sugois better than that on the Nashbars.
3) The Sugois are not cut straight down, like a pair of pants (and like the Nashbars). Instead, they're cut so that there's a slight bend at the knee, which reduces bunching and keeps them from sliding up/down on your leg.
The only downside to the Sugoi warmers is that they're shorter than my Nashbars (which are actually super long and cover my entire leg from ankle to groin). If I were 2" taller or more, I'd want to size up to the Small in these Sugois.
Overall, highly recommended.