Running shoes I can go between 10.5 (asics, brooks) and 11 (Newbalance, saucony, mizuno).
These were just a bit tight for me... a "racefit" that just got cramped after a while. (They didn't pack out as much as i was expecting)
I ended up getting a good deal on some boots from finndog, so I grabbed those and figured I'd sell which ever one fit better. The 27.5 that I got from him were a bit longer sole length and that made a difference.
Other than that, I honestly liked the boots alot. I felt like the cuff of my Lange Banshee's didn't conform to my leg well. What I liked most about the Diablo is that the powerstrap is like a 5th buckle that wraps the liner around your leg. It made for an awesome fit and awesome steering.
Without trying them on, I would say the most accurate measurement would be bootsole length. These Tecnica's are (I believe) a 304mm bootsole, but I will confirm that when I get home.