I recently picked up an old system with fairly noisy fans. I'd like to replace them, but the fans themselves don't have any mention of their size. Now, I know that there's standard sizes like 40, 60, 80, 92, 120 (and non standard ones, like sticking a box fan on the side of your system).
My question is, how do I measure the size? Is it by the size of the blades, or the square casing they're in?