I am 6'1" but weigh about 135 - 140 pounds. By height I would be a medium, but by weight a small. I know these need to fit right to get the benefit. Any sizing recommendation?

When I was buying these tights, I tested both the M & L (I'm 6'2"/155 lbs) because I often find the inseam to be too short on many tights. However, I found that these run slightly longer than other running tights, so the M was perfect for me because the L had way too large of a waist. As for you, I'd imagine that sizing by height is probably better than by weight since you're almost into the listed weight for a M. However, probably the best way to figure out what is best (if possible) is to order both the M & S and then only keep whichever one fits better.