My case has room for more fans for several sizes, and has these clip on type brackets that you just stick the fan into. How do I measure the brackets to know what kind of (size) fan to fit into them?
I'm pretty sure its the diagonal length. Usually you can tell by just looking at it, the large ones are 120mm, and the normal looking ones are 80mm/92mm, and the really small ones aren't that common.
Look at a couple of fans at newegg and look its dimension under the specification tab. Then measure the dimension of the fan space in your case. See which one match the size of the fan, then thats the one that you need to buy.