Good fit for tall people, if you're not too muscular in the shoulders - Rab Merino+ 120 Long Sleeve Zip Tee - Men's

I'm 6' 2", 180lbs, athletic build. I ordered a medium because a like a snug fit for base layers. The torso is form fitting but not tight; the arms have plenty of length. The collar is snug enough -- not too loose as one online review suggested, nor uncomfortably tight. If you have muscular arms/shoulders, the armpits fit very tightly. They don't restrict movement (the material has plenty of stretch), but there is literally no airspace between the fabric and my skin in any part of that area. It's an odd feeling, but not a deal-breaker for me.

The fabric seems very light and breathable. It may not provide total sun protection in harshly lit environments like summer snowfields; I haven't had a chance to evaluate that yet.