I have used cool wings for many races and they were a life saver for me both in terms of sunburn but also for cooling. Dry, they do nothing to cool you but aren't any hotter either. But wet, they help a ton. When I was racing long distance tri I would soak them with water from sponges at every aid station and that made a huge difference. Mountain bike racing I used to provision for extra water to pour onto my sleeves. They do make a huge difference if you can take advantage of evaporation.

I think someone else gave you the link to the sizing information. If you are unsure, cheat towards the next largest size. I believe I use large (I'm 5'7" but quite muscular through my arms), they are snug but the length is good.