Unibody Laptop Dual Drive limit?

What you are reading is the configuration sheet for the system as Apple sold the system. That is not the limitation on how large a HD you can put in.

The true limits is the physical size of the drive, the systems SATA speed limit and lastly, the limits of the OS version you are planning on using.

To make this easier, I would recommend you just replace your current HD with a larger unit and I would recommend going with a SSHD hybrid driver vs a standard HD.

I don't recommend dual drive setups in MacBook Pro's any more. They are more of a hassle than there worth. Many people wanted to expand their systems with SSD's to speed the boot up. Often the systems SATA ports wouldn't handle the higher speed of the SSD or if you flipped the drives around the original Apple drives lack of onboard crash guard ended up killing the HD drive (Apple uses logic on the main logic board and only for the primary SATA port). Which are the bigger issues not to do this.

Seagate makes a nice SSHD which IFIXIT sells that will work just fine in your system. As to the OS you should upgrade it to the newest your system can support and you might think about upgrading your RAM as well.