I bet you have an 80mm you know, they are awful! The best way to go is 180mm, but of course, not alot of cases have the ability to house 180mm, so the best next to that is 120mm..
If it looks kinda small (~3.5") it's 80mm, if it looks kinda big (~6") it's 120mm. Although I do thing there is a 92mm fan size also? But those seem to be far less common than 80 or 120.