If you're looking for just a decent base layer to help you stay cool and dry, this will do the job. The small was way too tight, so I got a medium, and I find it could be a little tighter. But it's a good one to add to the collection for summer rides.
I'm comparing these to the pearl izumi sun sleeves. The fit seems a bit more snug on these arm coolers, and they seem to be much cooler. I have these to protect my tattoo sleeve against the sun and they do the job, while keeping me cool. I also like how the cut on these is a little shorter on length of the arm compared to the pearl izumi. I'm 5'8, 158 lbs, my arms aren't Andy Schleck skinny but they're not big either. I got a size L/XL, and it's perfect. The S/M I could hardly fit on.