best back up hard drive?

[ManiacVirus]

Hi guys bare with me as I'm quite new to all this still.
I recently built a PC using the Coolermaster CM690 II Advanced case.
I need to sort out a back up hard drive and wondered if I could use my sata x dock as a way of connecting a back up drive.
If so would this be a good/cheap option and what sort of hard drive would you recommend?

 

[Surveyor]

Is the dock USB 2.0 or 3.0, or eSATA?

Does it fit 3.5" drives?

How big a drive do you need?