I wore an eco-mesh shirt last year on our trip. All 3 days, even in triple digits. Slept in it each night too. I also own several Adventure and Madison River shirts. I like the eco-mesh best but the Adventure is not much differnt, other than the cuffs and 3 top buttons instead of the single Velcro closure. Most of the time I leave the shirt opened. If I need more pockets, then it is the Madison River. Anyway, all my 3-season trips use some sort of Rail Riders.
The large fits me in sleeve length as a normal medium shirt, but is fairly loose around the body. I like the Napolean pocket, you can stuff a small camera in it and it stays put. The collar is about an inch taller than a t-shirt, and I don't notice it at all. In 3 season hiking, I wear the eco-mesh 90% of the time. I have thin arms, so the elastic is lose on my wrists. It is tight if I hike it up below my elbows... stays there but the elastic does not bother me. They would probably be too tight on your arms.
The shirt is light in weight, but the material feels thicker than it weighs, if that makes sense. Breathes well for me.
I have a bunch of OXT stuff, and I wear it lot around town, but it does not breath that well for me. I find the OXT gets stinky easily... at least my wife says so. When I want to hike in short sleeves, the Rail Riders Eco-speed-T breathes the best, but no long sleeves. It is rated UPF 20, but I really do not know what that means.
If I was running, I probably would not pick the eco-mesh. I sweat a lot more when running, and not sure I would like it. One that does interest me is the Golite Wildwood Trail Longsleeve. 4 oz in medium, supposedly wicks well. They are fitted sizing, so I would get one size larger, as I don't care for tight shirts. UPF is 50+. List price is $50. I have been looking at it for a while.
Edit: P.S. If you would like I can send you all 3 styles to try on. They are all larges.