From everything I've read the Opus 7's are a super aggressive boot. I tried on a few Celsius boots (Climate, Cirrus, and CLS) at my mtn side shop and they fit pretty true to size. I'm a 10.5 on the scale and a 10.5 is what fit the best. I have M/L Force and the Cirrus fit into them no problem. They actually have a smaller outer profile than my current size 10 vans. I would assume the Opus sizing is similar to the others.