It'd definitely smaller than 12". That's the size of the SFF PC I have on my desk, and this thing, next to the coffee cup and mouse shown, is definitely not as large. The subtitle of the article states, "HP Z2 Mini delivers server-grade power in a 2.3 by 8.5 inch package".
That makes sense. I'd suggest "diagonal" as opposed to "diameter", as "diameter" suggests that the measurement is being taken across the flats. I see what you're saying though. The Mac Mini was, I recall, of roughly the same area as a CD jewel case, which is consistent with the size you describe.
