Here's a link to it. I had to search around for a while because it's not in MidCurrent's "news" section, it's in the "ask the experts" section:
What’s the Right Length for a Trout Rod? | MidCurrent
Once I started reading it I realized I had read it before. It's been up on their site for a while.
I tend to agree with the Orvis guy who said that the 8' 6" is often the sweetest rod in any particular series, because it has the right combination of both length for line control and lightness for balance. That doesn't necessarily mean it will be the best for any particular purpose, though. As you go longer than that you are adding more material to the stiffest, heaviest part of the butt section, which makes for a clumsier rod, but you will gain reach and mending ability. As you go shorter you may gain finesse and delicacy, but you will lose distance.
Tom Morgan, the legendary former rod designer at Winston, offers his own thoughts on rod length and line size here:
Tom Morgan Rodsmiths - Yellowstone Country Water