Optiplex SX270 replacement hard drive

Hello, I recently purchased a used DELL Optiplex SX270 small (10"X10") PC at an auction thats missing the hard drive and need to know wich replecement hard drive would be the best to buy in regards to both the capacity as well as the physical size ..... I'm a bit confused after searching myself due to the fact that this model is considered a "desktop" but but it seems that only a "laptop" sized hard drive (2.5") would fit in the bay... Thank You In Advance For ANY Help Somebody Can Offer Me.