The limitation is physical size. While the dual drive carrier can only support a 2.5" drive which is not higher that the top of carrier frame.
So unlike many SSD's HD's are taller when you get larger storage space (larger than 750 GB). If you want you could go with a SSD like a Samsung 2 or 4 TB.
Otherwise OS-X/MacOS has no problem accessing larger drives until you get in the Exabyte sizes: Mac OS X: Mac OS Extended format (HFS Plus) volume and file limits
Now the rub! Some models of MacBook Pro's have issues supporting SATA III (6.0 Gb/s) drives. It makes no difference who's optical drive carrier you use as the issue is within the systems logic board. Review this: OWC Data Doubler to make sure your system is able to use a SATA III drive. If not you'll need to use a FIXED speed SATA II drive. This is where things get a bit messy. Fixed SATA II (3.0 Gb/s) HD's are getting hard to find. You will need to look out on the web to see if you can find one. Make sure you review the spec sheet for the given drive to make sure its a fixed speed drive.
For a long while we had auto sense drives like the older Seagate SSHD here's its spec sheet: ST1000LM014. As you can see in the line: Interface Access Speed (Gb/s) where all three SATA speeds are supported: 6.0/3.0/1.5 Sadly, Seagate no longer offers this drive. While you still can find one out in the wild it still won't work in your systems optical drive carrier as the auto speed technology will be confused. In addition the newer version of the Seagate SSHD the FireCuda Complete is only a fixed sped SATA III (6.0 Bb/s) drive and it too won't work in your system if it requires a SATA II drive.
This same technology is in many SSD's and likewise they won't work either in the optical drive carrier. This is why you need to find an older fixed speed SATA II (3.0 Gb/s) only drive for the carrier as outlined in the OWC Data Doubler dock you have one of the odd systems.
@xilitive - If your current drive is dead you won't have an issue with the SATA speed problem of the optical carrier as you don't need it here.
I would recommend going with the FireCuda drive I spoke about its a very good drive and you can go up to 2 TB if you need that much space.
Just make sure you don't have the retina series system.