I think I read in this forum that the chassis fan header was removed on some newer Optiplex's. One way to use an aftermarket fan is get one with a molex (4-pin) connector.  Get a SATA power splitter.  Then one connection can go back to DVD drive and in the other plug in a SATA power to molex adapter to plug the fan into. I haven't seen any fans yet that can just simply plug into SATA power.  They're either molex or are meant to *plug into MB.*  *Some require an adapter. As for fan size, it looks like you may have measured already?  Fans sizes typically round to the nearest zero, whether it be 60mm, 80mm, etc.  I'm sure there are a few sizes in between, like 92mm for one.
Hi guys,  I notice there is a front fan mount on the chassis, is it a 60mm system fan that it takes? There doesn't appear to be a another fan header on the board so how would you go about powering it? Thanks. 
OptiPlex 5050 SFF Front Chassis Fan