It probably would be able to fit. Most gtx 660 are about 9 inches which isn't very big but your case is kind of small. What case do you have? A few gtx 660 are small I would just get one of those like this one http://www.newegg.com/Product/Product.aspx?Item=N82E16814127707 It's only 8.66 inches long.
gpu sizes and how cases are built are never the same. You have to post a specific case and a specific gpu. some gpus are oversized with a good cooler, some cases have removable HDD cages/ unique way of hdd mounting that doesnt interfere with gpu legnth(for example, my xclio 767 mounts hdds on its side, giving it full gpu legnth for the case.