30.5cm x 27.1cm to be specific, or 12" by 10.7" if my conversion skills are correct.
I have a define R5 for my current/old build, and while I do like it, what sets apart some of the other choices on the list is the 4x USB 3.0 on the front. I'm also less a fan of the design of it, it's functional and wonderful, but better airflow and design can be achieved with other cases. I do appreciate the suggestion as they are mighty fine cases.