I noticed it was smaller, but it will still fit and work properly?
If it's the Wix made ST9688 it should fit fine. Work properly? Can't think of reason why it wouldn't. As noted the ST9688 OD is smaller, however it is slightly longer than a spec 9688 application. Overall oil volume close to spec 9688.
If you want to see pics with a comparison ST9688(7317) vs spec 9688 click on linked thread. Ignore the comments regarding what the 7317 would look like, it's an early new Wix ST thread. The 7317 application and 9688 consolidated and called the ST9688. I'd add, much better imo than using the shorty 6607 application which many quick lubes and indies use for the Hyun/Kia 9688 application. Beyond that, if not satisfied with answer better use spec size. https://bobistheoilguy.com/forums/ubbthreads.php/topics/4024307/all/Supertech_ST9688